Uttam Sinha Mahapatra

University of Calcutta, Kolkata, Bengal, India

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Publications (55)134.04 Total impact

  • Sudip Chattopadhyay, Uttam Sinha Mahapatra, Rajat K. Chaudhuri
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    ABSTRACT: We have computed the potential energy surfaces (PESs) of F2, Cl2, Br2, and I2 using the size-extensive state specific multireference coupled cluster (SS-MRCC) method. The MR character of the system considered here at large distances and the presence of low-lying intruder states are known to be the major causes of incorrect or inaccurate predictions of the PES. The SS-MRCC theory is tailored to treat degeneracies of varying extent while bypassing the intruder problem. The quality of the computed PES has been gauged by computing spectroscopic constants. The calculated properties show a good agreement with available experimental data and the errors in the calculated molecular properties compare favourably with the most elaborate current-generation calculations of the literature. The accuracy of the computed PES of F2 is such that it has been proved to calculate the vibrational spectrum of the 22 levels with a minimum and maximum absolute deviation of 2 and 57 cm−1, respectively, from the experimental values. The highly satisfactory performance of the SS-MRCC method, vis-a-vis the other sophisticated methods, in describing the vibrational levels is noticeable for one of the more difficult systems such as F2 clearly indicates that the present method is reliable in studying the vibrational energy levels.
    Molecular Physics 10/2014; 112(20). · 1.67 Impact Factor
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    ABSTRACT: The coupled cluster based linear response theory (CCLRT) with four-component relativistic spinors is employed to compute the electric field gradients (EFG) of $^{35}$Cl, $^{79}$Br, and $^{127}$I nuclei. The EFGs resulting from these calculations are combined with experimental nuclear quadrupole coupling constants (NQCC) to determine the nuclear quadrupole moments (NQM), $Q$ of the halide nuclei. Our estimated NQMs [$^{35}$Cl=-81.12 mb, $^{79}$Br = 307.98 mb, and $^{127}$I = -688.22 mb] agree well with the new atomic values [$^{35}$Cl =-81.1(1.2), $^{79}$Br= 302(5), and $^{127}$I =-680(10) mb] obtained via Fock space multireference coupled cluster method with Dirac-Coulomb-Breit Hamiltonian. Although our estimated Q($^{79}$Br) value deviates from the accepted reference value of 313(3) mb, it agrees well with the recently recommended value, Q($^{79}$Br) = 308.7(20) mb. Good agreement with current reference data indicates the accuracy of the proposed value for these halogen nuclei and lends credence to the results obtained via CCLRT approach. The electron affinities yielded by this method with no extra cost, are also in good agreement with experimental values which bolster our belief that the NQMs values for halogen nuclei derived here are reliable.
    The Journal of Physical Chemistry A 10/2013; · 2.77 Impact Factor
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    ABSTRACT: Theoretical investigations of the super-heavy elements (SHEs) are extremely challenging and are often the sole source of useful chemical information. Relativistic Fock-space multireference coupled cluster (RFS-MRCC) computations have been carried out for evaluating the ionization potential (IP), excitation energies (EE), nuclear magnetic hyperfine constant ($A$), lifetime ($\tau$) and Land{\'e} $g$ factor of eka-lead (Fl). To judge the accuracy of Fl results, similar calculations are performed for Pb, which shows a nice and consistent agreement with known experimental values. Thus, we believe that our predictions for Fl are reliable and useful for the simulation of experimental behavior. To the best of our knowledge, no prior theoretical and/or experimental information is available for $A$, $\tau$ and g-factor of this SHE. The higher IPs and EEs of Fl with respect to Pb, indicate the former to be more inert and less metallic than Pb. This is contingent on the effects of the relativistic stabilization of the $7s$ and $7p_{1/2}$ orbitals. The present analysis demonstrates the influence of higher-body cluster operators on atomic properties. The close agreement with experiment (having an estimated error within 1-2\%) indicates that the FS-MRCC method is a reliable predictive tool in cases where the experimental results are not readily available, such as the SHEs. The remaining source of error possibly stems out from the omission of the full-blown triple virtual excitations and the absence of Breit interaction.
    The Journal of Physical Chemistry A 05/2013; · 2.77 Impact Factor
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    ABSTRACT: Optimized geometries and normal mode frequencies are evaluated for the ground and low lying excited states of {\em cis-}stilbene ($c$S), {\em trans-}stilbene ($t$S), and 4a,4b-di-hydro-phenanthrene (DHP) from calculations performed with the improved virtual orbital complete active space configuration interaction (IVO-CASCI) method. Structural parameters and vibrational frequencies are also provided for the non-planar conformation of {\em trans-} stilbene. The ground and low lying excited state vibrational frequencies of these systems are found to be real, thus confirming the stability of their optimized geometries. The calculations further show that a non-planar conformer of {\em trans-}stilbene is the most stable among these four systems. The calculated ground and low lying excited state geometries and vibrational frequencies agree well with experiment and with prior theoretical estimates where available. Our IVO-CASCI based multi-reference M{\"o}ller-Plesset (MRMP) computations place the $^1$B$_u$ state of {\em trans} stilbene to be $\sim$4.0 eV above the ground X$^1$A$_g$ state, which is in accord with experiment and with earlier theoretical estimates. The 1$^1$B$_u$ state of {\em trans-}stilbene can be represented by the highest occupied molecular orbital (HOMO)$\rightarrow$lowest unoccupied molecular orbital (LUMO) transition (ionic type) from the ground state, whereas its 2$^1$B$_u$ state is dominated by the HOMO$\rightarrow$ LUMO+1 and HOMO-1$\rightarrow$LUMO transitions (covalent type). Likewise, the 1$^1$B and 2$^1$B states of {\em cis-}stilbene and DHP are also found to be ionic and covalent types, respectively. Multi-reference M{\"o}ller- Plesset (MRMP) perturbation method calculations (based on IVO-CASCI reference states) are presented for the torsional potential energy curve connecting the {\em trans}$\rightarrow${\em cis}$\rightarrow$DHP ground state minima.
    The Journal of Physical Chemistry A 03/2013; · 2.77 Impact Factor
  • Rajat K. Chaudhuri, Sudip Chattopadhyay, Uttam Sinha Mahapatra
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    ABSTRACT: We report the plasma screening effect on the first ionization potential (IP) and [He]2s{sup 2} ({sup 1}S{sub 0}){yields}[He]2s2p/2s3p allowed ({sup 1}P{sub 1}) and inter-combination transitions ({sup 3}P{sub 1}) in some selected Be-like ions. In addition, we investigate the spectral properties of [Ne]3s ({sup 2}S{sub 1/2}){yields}[Ne]np ({sup 2}P{sub 1/2} and {sup 2}P{sub 3/2} for n = 3, 4) transitions in Ca X and Fe XVI ions (Na I isoelectronic sequence) and [He]3s({sup 2}S{sub 1/2}){yields}[He]np ({sup 2}P{sub 1/2} and {sup 2}P{sub 3/2} for n = 2, 3) transitions in Li, B II, and N IV (Li I isoelectronic sequence) under plasma environment. The state-of-the-art relativistic coupled cluster calculations using the Debye model of plasma for electron-nucleus interaction show that (a) the ionization potential decreases sharply with increasing plasma strength and (b) the gap between the [He]2s{sup 2} ({sup 1}S{sub 0}){yields}[He]2s2p({sup 1,3}P{sub 1}) energy levels increases with increasing plasma potential and nuclear charge. It is found that the [He]2s{sup 2} ({sup 1}S{sub 0}){yields}2s3p ({sup 1,3}P{sub 1}) transition energy decreases uniformly with increasing plasma potential and nuclear charge. In other words, the spectral lines associated with 2s-2p (i.e., {Delta}n=0, where n corresponds to principle quantum number) transitions in Be I isoelectronic sequence exhibit a blue-shift (except for Be I, B II, and the lowest inter-combination line in C III, which exhibit a red-shift), whereas those associated with 2s-3p (i.e., {Delta}n{ne}0) transitions are red-shifted. Similar trend is observed in Li I and Na I isoelectronic sequences, where spectral lines associated with {Delta}n=0 ({Delta}n{ne}0) are blue-shifted (red-shifted). The effect of Coulomb screening on the spectral lines of ions subjected to plasma is also addressed.
    Physics of Plasmas 08/2012; 19(8). · 2.38 Impact Factor
  • Sudip Chattopadhyay, Uttam Sinha Mahapatra, Rajat K. Chaudhuri
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    ABSTRACT: We provide further tests and illustrations of the complete active space based state specific multireference Møller–Plesset perturbation theory (SS-MRMPPT) which opens the way for the treatment of dynamic correlations in situations containing significant static correlations also in an accurate, size- extensive and intruder free manner enjoying at the same time a very favorable computational cost. We have investigated various interesting and computationally challenging systems [H2O, H2O+, BeC, MgC, CO+, Be3, benzene, trimethylenemethane and 1,2,3-tridehydrobenzene] in either their ground, or excited/ionized states. It is found that SS-MRMPPT calculations provide very encouraging results which can be meaningfully compared with other state-of-the-art theoretical estimates. Present results convincingly indicate that the SS-MRMPPT method is not only successful in portraying situations that warrant MR description but also performs acceptably good in cases where a naive single-reference method is enough which reinforces the claim that the SS-MRMPPT is a very useful and flexible ab initio method.
    Chemical Physics 06/2012; · 1.96 Impact Factor
  • Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: We have tested the linked version of a iterative (partial) triples correction for the Jeziorski-Monkhorst ansatz based state-specific multireference coupled cluster (SS-MRCC) approach with singles and doubles (SD) excitations [abbreviated as SS-MRCCSDT-1a and SS-MRCCSDT-1a+d]. The assessments of SS-MRCCSDT-1a and SS-MRCCSDT-1a+d schemes have been performed on the ground potential energy surface (PES) of P4, Li(2),Be(2) systems which demand the MR description, and on study of the excitation energy between the ground and first excited state for P4 system. Illustrations in the isomerization of cyclobutadiene also show the power of the schemes. One of the designed features of the SS-MRCCSDT-n methods introduced here is that they do not require storage of the triples amplitudes. In the entire range of geometries, we found a definite improvement provided by SS-MRCC with SDT-1a and SDT-1a+d schemes over the standard SD one. In the nondegenerate regions of PES, the closeness of the performance of the single-reference CC to the SS-MRCC methods increases after inclusion of even partial triple excitations. Generally, the performance of the SS-MRCCSDT-1a+d approach is closer to the corresponding full configuration interaction (FCI) one than to the SS-MRCCSDT-1a specially in the degenerate geometries (as is evident from nonparallelism error). The deviation from FCI for the first excited state of the P4 model using various SS-MRCC theories with different truncation schemes obtained by converging on the second root of the effective Hamiltonian has also been reported. We also compare our results with the current generation state-of-the-art single and multireference CC calculations to envisage the usefulness of the present approach. Initial implementation indicates that the SS-MRCCSDT-n formalism can provide not only reliable excitation energies and barrier height even when used in a relatively small model space, but also offers a considerable promise in generating the entire energy surface with low nonparallelity error.
    Journal of Computational Chemistry 03/2012; 33(14):1285-303. · 3.84 Impact Factor
  • Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: We apply here a new iterative triples correction for the single reference coupled-cluster method with singles and doubles scheme (termed as CCSDT-1a+d). All diagonal terms in have been included in the T3 determining equation, in addition to . This minor change in the CCSDT-1a approximation (includes the primary effects of connected triples excitations iteratively) shows a dramatic change in the study of correlation of weakly bonded systems as discussed here. To achieve a general assessment of the potentiality of the CCSDT-1a + d calculations, the ground state potential energy surfaces (PESs) of alkaline-earth dimers, including Mg2, Ca2 and the challenging Be2 are considered. The systems considered here are treated as four-electron systems, keeping other electrons frozen in the correlation calculations. Generation of PESs for these dimers is very sensitive to the level of theory employed for the electron correlation. Basic spectroscopic constants including dissociation energy extracted from the calculated ground state PES are also reported. For all systems under study, the overall performance of CCSDT-1a+d is very competitive with that of the CCSD(T)/CCSDT method, and better than that of CCSDT-1a indicating that the former approach is more effective in handling dynamic correlation effects in comparison to the latter one in the presence of quasi-degeneracy to different extents. To calibrate our results we also report frozen-core FCI calculations.
    Molecular Physics 01/2012; 110(2):75-83. · 1.67 Impact Factor
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    ABSTRACT: Highly accurate electronic-structure calculations for metastable electronic excited states are needed to supplement scant experimental data in screening possible systems for new higher-precision atomic clocks. We test the suitability of relativistic coupled-cluster methods for the low-lying 2Fo excited states of the Yb II ion by computing the ionization potentials (IP) for the 2S1/2 and 2Fo states of Yb I and the , j = 5/2, 7/2, electric octupole transition amplitudes. The calculations establish a minimum lifetime of six years and of 10− 1 s for the 2Fo7/2 and 2Fo5/2 states, respectively. In addition, computations for the lifetimes (τ) of its [Xe]4f146p(2Po) states are compared with high-precision experimental data as tests of the accuracy of our predictions. To our knowledge, this is the first relativistic ab initio estimate of the lifetime and ionization potential for the 2Fo states of Yb II, and the results demonstrate the suitability of these methods to aid in screening other candidates for atomic clocks.
    EPL (Europhysics Letters) 01/2012; 98(2). · 2.26 Impact Factor
  • Sudip Chattopadhyay, Rajat K Chaudhuri, Uttam Sinha Mahapatra
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    ABSTRACT: We present improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) and IVO-CASCI-based multireference Møller-Plesset perturbation theory (MRMPPT) calculations with an aim to elucidate the electronic structure of tetramethyleneethane (TME) in its lowest singlet and triplet state and to quantify their order and extent of splitting. The potential surfaces of singlet and triplet states for the twisting of TME are also studied. We found that the triplet state is higher in energy than the singlet one in the whole range of twisting angles with the energy gap minimum at a twisting angle of about 45°. Harmonic vibrational frequencies of TME have also been calculated for both the states. We also report the ground to first excited triplet state transition energies. Our results are analyzed with respect to the results available in the literature to illustrate the efficacy of our methods employed. We also demonstrate that the spin character of the ground state of disjoint, TME-like diradicals can be manipulated by using appropriate selection of annulenic spacer to separate the allyl groups of TME.
    ChemPhysChem 10/2011; 12(15):2791-7. · 3.35 Impact Factor
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    ABSTRACT: Due to its flexibility and possible systematic improvement, the Fock-space (FS) multireference coupled-cluster (MRCC) method remains a very important tool for the computation of energy differences of spectroscopic interest. In the present work, the FS MRCC method for the electron detachment process has been applied to determine the magnetic hyperfine constant A{sub J} and nuclear quadrupole moments Q (related to electric hyperfine constant B{sub J}) for the lowest multiplets of {sup 33}S{sup -}, {sup 35}Cl, and {sup 37}Cl with Dirac-Fock orbitals. In addition, we also report {sup 2}P{sub 3/2}([Ne]3s{sup 2}3p{sup 5}) {yields} {sup 2}P{sub 1/2}([Ne]3s{sup 2}3p{sup 5}) magnetic dipole transition matrix element and electron affinity of {sup 35}Cl (i.e., ionization energy of Cl{sup -}). Calculated properties are in very good agreement with the available new standard or reference values.
    Physical Review A 10/2011; 84(4). · 3.04 Impact Factor
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    ABSTRACT: The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is applied to determine the geometries of the ground state of free-base porphin and its metal derivatives, magnesium and zinc porphyrins. The vertical excitation energies and ionization potentials are computed at these optimized geometries using an IVO-based version of multireference Möller-Plesset (IVO-MRMP) perturbation theory. The geometries and excitation energies obtained from the IVO-CASCI and IVO-MRMP methods agree well with experiment and with other correlated many-body methods. We also provide the ground state vibrational frequencies for free-base porphin and Mg-porphyrin. All frequencies are real in contrast to self-consistent field treatments which yield an imaginary frequency. Ground state normal mode frequencies (scaled) of free-base porphin and magnesium porphyrin from IVO-CASCI and complete active space self-consistent field methods are quite similar and are consistent with Becke-Slater-Hartree-Fock exchange and Lee-Yang-Parr correlation density functional theory calculations and with experiment. In addition, geometries are determined for low-lying excited state triplets and for positive ion states of the molecules. To our knowledge, no prior experimental and theoretical data are available for these excited state geometries of magnesium and zinc porphyrins. Given that the IVO-CASCI and IVO-MRMP computed geometries and excitation energies agree favorably with experiment and with available theoretical data, our predicted excited state geometries should be equally accurate.
    The Journal of Chemical Physics 08/2011; 135(8):084118. · 3.12 Impact Factor
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    ABSTRACT: Ionization potential and low lying 1S01P1 excitation energies (EE) of highly stripped He-like ions C4 +, Al11 +, and Ar16 + embedded in plasma environment are calculated for the first time using the state-of-the-art coupled cluster (CC)-based linear response theory (LRT) with the four-component relativistic spinors and compared with available experimental data from laser plasma experiments. Debye's screening model is used to estimate the effect of plasma on the ions within the relativistic and non-relativistic framework. The transition energies computed at the CCLRT level using the Debye model agree well with experiment and with other available theoretical data. To our knowledge, no prior CCLRT calculations within the Dirac–Fock framework are available for these systems. Our calculated transition energies for helium-like ions are in accord with experiment; we trust that our predicted EE might be acceptably good for the systems considered. Our preliminary result indicates that CCLRT with the four-component relativistic spinors appears to be a valuable tool for studying the atomic systems where accurate treatments of correlation effects play a crucial role in shaping the spectral lines of ions subjected to plasma environment.
    Journal of Physics B Atomic Molecular and Optical Physics 07/2011; 44(16):165701. · 2.03 Impact Factor
  • Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: Recently, Mukherjee and co-workers (2008 Chem. Phys. 349, 115) have proposed the uncoupled state-specific multireference coupled cluster approach (UC-SSMRCC), attuned particularly for the multireference systems to provide a relaxed description of the nondynamical correlation (stemming from quasidegeneracy) in the presence of dynamical correlation. By invoking an analogue of the anonymous parentage approximation in the coupling terms of the state-specific multireference coupled cluster (SS-MRCC) theory one obtains the UC-SSMRCC method without significantly sacrificing the accuracy and simultaneously retains all the key features of the parent theory. For this very reason the UC-SSMRCC calculations using large basis sets are computationally more feasible even with the inclusion of connected triples than the parent one. As far as our knowledge is concerned, no application of either the parent SS-MRCC or UC-SSMRCC has been reported in weakly bound systems to date. Hence, we have performed a UC-SSMRCC study of a weakly bound dimer (say X1Σ+ Be2) by the singles-, doubles-, triples-1a (SDT-1a) level of approximation. Because of the strong near-degeneracy of 2s and 2p atomic orbitals, X1Σ+ Be2 is a very suitable benchmark problem for quantum chemistry and a critical test for new theoretical methods and procedures. This study reveals that the quality of the description of X1Σ+ Be2 critically depends on the level of truncation of the cluster operators (and also on the choice of basis set) and on a delicately poised treatment of dynamic and nondynamic correlation effects. Our computed spectroscopic constants are in acceptably good agreement with other previously reported current generation correlation recovery theoretical estimates, indicating that the UC-SSMRCC method with SDT-1a is capable of achieving a qualitatively correct description of the X1Σ+ Be2 energy surface even with the smallest reference space. The UC-SSMRCC scheme is also capable of providing estimates with almost no deterioration of the quality vis-á-vis the parent SS-MRCC. Accordingly, it is hoped that the realm of the application of the UC-SSMRCC method will be increased.
    Journal of Physics B Atomic Molecular and Optical Physics 04/2011; 44(10):105102. · 2.03 Impact Factor
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    ABSTRACT: In view of its importance in high precision spectroscopy, the valence universal multireference coupled cluster (VU-MRCC) method with four-component relativistic spinors has been applied to compute ionization potential (IP) and excitation energies (EEs) of the indium atom (In I). The effect of electron correlations on the ground and excited state properties is investigated using different levels of CC approximations and basis sets. This study reveals that for a given basis, the linearized VU-MRCC method tends to underestimate the IP, EEs and other one-electron properties such as magnetic hyperfine constant (A) compared to the full blown VU-MRCC method. Our computed results have been compared with available theoretical and experimental data. The IP, EEs, A and oscillator strengths (f) determined at the VU-MRCC level are in excellent agreement with the experimental results. The properties reported here further demonstrate that a basis set with at least h-type of orbitals is ubiquitous to achieve converged results.
    Journal of Physics B Atomic Molecular and Optical Physics 03/2011; 44(6):065003. · 2.03 Impact Factor
  • Uttam Sinha Mahapatra, Sudip Chattopadhyay, Rajat K Chaudhuri
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    ABSTRACT: The complete active space spin-free state-specific multireference Møller-Plesset perturbation theory (SS-MRMPPT) based on the Rayleigh-Schrödinger expansion has proved to be very successful in describing electronic states of model and real molecular systems with predictive accuracy. The SS-MRMPPT method (which deals with one state while using a multiconfigurational reference wave function) is designed to avoid intruder effects along with a balanced description of both dynamic and static correlations in a size-extensive manner, which allows us to produce accurate potential energy surfaces (PESs) with a correct shape in bond-breaking processes. The SS-MRMPPT method is size consistent when localized orbitals on each fragment are used. The intruder state(s) almost inevitably interfere when computing the PESs involving the breaking of genuine chemical bonds. In such situations, the traditional effective Hamiltonian formalism often goes down, so that no physically acceptable solution can be obtained. In this work, we continue our analysis of the SS-MRMPPT method for systems and phenomena that cannot be described either with the conventional single-reference approach or effective Hamiltonian-based traditional MR methods. In this article, we investigate whether the encouraging results we have obtained at the SS-MRMPPT level in the study of cis-trans isomerization of diimide (N2H2), ethylene (C2H4), and 1,3-butadiene (C4H6) carry over to the study of chemical reactions. The energy surfaces of the double-bond flipping interconversion of the two equivalent ground and two lowest singlet state structures of cyclobutadiene have also been studied. All results have been discussed and assessed by comparing with other state-of-the-art calculations and corresponding experimental data whenever available.
    Journal of Computational Chemistry 01/2011; 32(2):325-37. · 3.84 Impact Factor
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    Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: The complete model space (CAS) based "genuine" single root multireference (MR) coupled cluster (sr-MRCC) method [Mahapatra and Chattopadhyay, J. Chem. Phys. 133, 074102 (2010)] has been extended to enable geometry optimizations by adopting the numerical gradient scheme. The sr-MRCC theory is designed to treat quasidegeneracies of varying degrees through the computation of essential static and dynamic correlation effects in a balanced way while bypassing the intruder states problem in a size-extensive manner. The efficacy of our sr-MRCC gradient approach has been illustrated by the optimization of the geometries of N(2)H(2),CH(2),C(2)H(4),C(4)H(4),O(3) as well as trimethylenemethane (TMM) molecular systems, since such cases, by virtue of their complexity, warrant truly multireference description. We have explored the capability of the sr-MRCC approach to yield rotational energy surfaces for the ground and first singlet excited states of N(2)H(2). We also intend to explore the ground and the excited state energetics of some model systems (such as P4, H4, and H(8)) for the computation of excitation energies by relying on the sr-MRCC method. An analysis of the results and a comparison with previous pertinent theoretical works including state specific MRCC (SS-MRCC) theory of Mukherjee and co-workers have also been presented. Although in most of the cases, we observe a close behavior between the sr-MRCC and SS-MRCC method, the error in the sr-MRCC is lower than the overall error of the SS-MRCC calculations in the vicinity of the transition region (manifesting a significant quasidegenerate character). The present results show that the sr-MRCC method and its numerical gradient variant are generally applicable to very demanding model and realistic chemical problems at acceptable accuracy and affordable computational expense which together attests the efficacy and viability of the sr-MRCC formalism for handling of static and dynamic correlations simultaneously thereby ensuring a balanced description for bond-breaking and other quasidegenerate situations with a various degree of MR character. Our preliminary results illustrate that our sr-MRCC method is a potential competitor for other state specific MRCC theories.
    The Journal of Chemical Physics 01/2011; 134(4):044113. · 3.12 Impact Factor
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    Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: We have employed complete active space based single root multireference coupled cluster method (the resulting method is referred to by the acronym sr-MRCC) to compute the potential energy surfaces (PESs) of some well studied "protypical model" systems for which a highly accurate and reliable database is available for comparison. As that of state-specific theory, the sr-MRCC approach focuses and correlates one state while using a multiconfigurational reference and thus it naturally avoids intruder states. The present method is structurally different from the well known state specific multireference coupled cluster (SS-MRCC) method introduced by Mahapatra et al. [Mol. Phys. 94, 157 (1998)]. As that of the SS-MRCC theory, the present method is also based on the Jeziorski-Monkhorst ansatz where a different exponential cluster operator exp(T(mu)) acts on its corresponding model function phi(mu). The final cluster finding equations contain coupling between the cluster operators for all the mu, which are mainly responsible to prove the extensivity of both the cluster amplitudes and the energy. The present sr-MRCC theory is size-extensive and size-consistent when localized orbitals are used. The systems considered here exhibit varying degrees of degeneracy at different regions of PES. The treatment of these systems via traditional effective Hamiltonian based methods suffers from divergence problems in the iterative solution of the CC equations (the issue termed as "intruder state"). The sr-MRCC results lie closer to the ones obtained by the SS-MRCC method for these systems. To judge the efficacy of the present method, we have compared our results with other previously published theoretical estimations, which clearly indicate that the present method is reliable in studying the dissociation PES of states plagued by electronic degeneracy as well as notorious intruder effects. The highly satisfactory performance of the sr-MRCC method, vis-a-vis the other sophisticated methods, in describing the lowest and the first excited singlet states of BeH(2) at points of high degeneracy is noticeable.
    The Journal of Chemical Physics 08/2010; 133(7):074102. · 3.12 Impact Factor
  • Uttam Sinha Mahapatra, Sudip Chattopadhyay
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    ABSTRACT: We have applied multireference (MR) perturbation theory to a degenerate case using the state-specific or single root MR approach (SS-MRPT). The objective of this paper is to provide an account of the applications of both Brillouin-Wigner and Rayleigh-Schrödinger versions of the SS-MRPT with Möller-Plesset partitioning of the hamiltonian. The method is very effective to provide an accurate and balanced treatment of dynamical and non-dynamical correlations over a wide range of geometrical parameters of the molecule under study, and hence, seeks to address bond breaking process in an accurate manner. Studies on dissociation potential energy curve and spectroscopic constants of the ground state of F2 are carried out to gauge its accuracy. F2 is a particularly challenging system for electronic structure theory due to strong dynamical and non-dynamical correlation effects. The calculations are compared with other available theoretical results and experimental spectroscopic data. The accuracy of the results appears to be encouraging, taking into account the low computational cost which supports the effectiveness of the method.
    06/2010: pages 31-42;
  • Uttam Sinha Mahapatra
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    ABSTRACT: The size-extensive second-order state-specific (or single root) multireference (MR) perturbation theory (SS-MRPT) in the Brillouin−Wigner (BW) form using Mller−Plesset perturbative evaluations of orders up to 2 [termed as SS-MRMPPT(BW)] presents a viable, as well as promising, approach to include both nondynamic and dynamic correlations in the study of the bond-stretching (in multireference/quasidegenerate situations) of molecular species with a manageable cost/accuracy ratio. It combines numerical stability in the presence of an intruder state problem with strict size consistency (when localized orbitals are used). In this paper, the SS-MRMPPT(BW) method has been shown to properly break the bonds (in the ground state) of several diatomic molecules (such as F2, Cl2 and Br2, and BH) that have posed a severe challenge to any many-body theoretical approach due to the presence of quasidegeneracy of varying degrees in the ground state. A comparison of the resulting potentials with the various theoretical results reveals that the method represents a valuable tool that is capable of properly accounting even for very strong quasidegeneracies, while also performing well in nondegenerate situations. In this work, we have also calculated spectroscopic constants (such as equilibrium bond lengths, vibrational frequencies, and dissociation energies) of the ground state of these molecular systems. The SS-MRMPPT spectroscopic constants are compared with the most accurate available ab initio calculations and other theoretical estimates of previous works to calibrate the efficacy of the method. For the sake of completeness, we also compare the computed spectroscopic constants with the experimental observations. The accuracy of computed spectroscopic parameters appears to be rather consistent over a multitude of systems for various basis sets. The SS-MRMPPT enables quantitatively accurate and computationally affordable analysis of potential energy surfaces and spectroscopic constants of various multireference systems in the ground state. It is particularly visible for spectroscopic parameters and nonparallelism error (NPE) calculations. The calculations further reveal that the SS-MRMPPT(BW) method compared to its Rayleigh−Schrödinger counterpart [SS-MRMPPT(RS)] provides a more accurate and consistent solution for the whole dissociation path and spectroscopic constants.
    Journal of Chemical Theory and Computation - J CHEM THEORY COMPUT. 02/2010;

Publication Stats

529 Citations
134.04 Total Impact Points


  • 2014
    • University of Calcutta
      • Department of Physics
      Kolkata, Bengal, India
  • 2011–2012
    • Maulana Azad College
      Aurangâbâd, Mahārāshtra, India
  • 2009–2011
    • Bengal Engineering and Science University
      • Department of Chemistry
      Hāora, Bengal, India
  • 2008–2011
    • Indian Institute of Astrophysics
      Bengalūru, Karnātaka, India
  • 2003–2006
    • Darjeeling Government College
      Rdorje gling, Bengal, India
  • 1998–2004
    • Indian Association for the Cultivation of Science
      • Department of Physical Chemistry
      Calcutta, Bengal, India