Publications (304)805.5 Total impact

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ABSTRACT: The DIIS (direct inversion of the iterative subspace) convergence acceleration algorithm is used in most electronic structure programs to solve the nonlinear coupled cluster amplitude equations. When DIIS is used the storage of previous trial vectors may become a bottleneck and discarding trial vectors may lead to a degradation of the convergence or even divergence. We discuss an alternative way of storing information from trial vectors where only the last 3 trial vectors are needed to maintain the convergence of the full set of previous trial vectors and which only requires minor modifications of an existing DIIS code.Journal of Chemical Theory and Computation 04/2015; 11(4):150403071611007. DOI:10.1021/ct501114q · 5.31 Impact Factor 
Article: Molecular response properties from a Hermitian eigenvalue equation for a timeperiodic Hamiltonian.
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ABSTRACT: The timedependent Schrödinger equation for a timeperiodic perturbation is recasted into a Hermitian eigenvalue equation, where the quasienergy is an eigenvalue and the timeperiodic regular wave function an eigenstate. From this Hermitian eigenvalue equation, a rigorous and transparent formulation of response function theory is developed where (i) molecular properties are defined as derivatives of the quasienergy with respect to perturbation strengths, (ii) the quasienergy can be determined from the timeperiodic regular wave function using a variational principle or via projection, and (iii) the parametrization of the unperturbed state can differ from the parametrization of the time evolution of this state. This development brings the definition of molecular properties and their determination on par for static and timeperiodic perturbations and removes inaccuracies and inconsistencies of previous response function theory formulations. The development where the parametrization of the unperturbed state and its time evolution may differ also extends the range of the wave function models for which response functions can be determined. The simplicity and universality of the presented formulation is illustrated by applying it to the configuration interaction (CI) and the coupled cluster (CC) wave function models and by introducing a new modelthe coupled cluster configuration interaction (CCCI) modelwhere a coupled cluster exponential parametrization is used for the unperturbed state and a linear parametrization for its time evolution. For static perturbations, the CCCI response functions are shown to be the analytical analogues of the static molecular properties obtained from finite field equationofmotion coupled cluster (EOMCC) energy calculations. The structural similarities and differences between the CI, CC, and CCCI response functions are also discussed with emphasis on linear versus nonlinear parametrizations and the sizeextensivity of the obtained molecular properties.The Journal of Chemical Physics 03/2015; 142(11):114109. DOI:10.1063/1.4913364 · 3.12 Impact Factor 
Article: The same number of optimized parameters scheme for determining intermolecular interaction energies.
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ABSTRACT: We propose the Same Number Of Optimized Parameters (SNOOP) scheme as an alternative to the counterpoise method for treating basis set superposition errors in calculations of intermolecular interaction energies. The key point of the SNOOP scheme is to enforce that the number of optimized wave function parameters for the noninteracting system is the same as for the interacting system. This ensures a delicate balance between the quality of the monomer and dimer finite basis set calculations. We compare the SNOOP scheme to the uncorrected and counterpoise schemes theoretically as well as numerically. Numerical results for secondorder MøllerPlesset perturbation theory (MP2) and coupledcluster with single, double, and approximate triple excitations (CCSD(T)) show that the SNOOP scheme in general outperforms the uncorrected and counterpoise approaches. Furthermore, we show that SNOOP interaction energies calculated using a given basis set are of similar quality as those determined by basis set extrapolation of counterpoisecorrected results obtained at a similar computational cost.The Journal of Chemical Physics 03/2015; 142(11):114116. DOI:10.1063/1.4915141 · 3.12 Impact Factor 
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ABSTRACT: Recently, we proposed a novel Lagrangianbased perturbation seriesthe CCSD(Tn) serieswhich systematically corrects the coupled cluster singles and doubles (CCSD) energy in orders of the MøllerPlesset fluctuation potential for effects due to triple excitations. In the present study, we report numerical results for the CCSD(Tn) series up through fourth order which show the predicted convergence trend throughout the series towards the energy of its target, the coupled cluster singles, doubles, and triples (CCSDT) model. Since effects due to the relaxation of the CCSD singles and doubles amplitudes enter the CCSD(Tn) series at fourth order (the CCSD(T4) model), we are able to separate these effects from the total energy correction and thereby emphasize their crucial importance. Furthermore, we illustrate how the ΛCCSD[T]/(T) and CCSD[T]/(T) models, which in slightly different manners augment the CCSD energy by the [T] and (T) corrections rationalized from manybody perturbation theory, may be viewed as approximations to the secondorder CCSD(T2) model. From numerical comparisons with the CCSD(Tn) models, we show that the extraordinary performance of the ΛCCSD[T]/(T) and CCSD[T]/(T) models relies on fortuitous, yet rather consistent, cancellations of errors. As a side product of our investigations, we are led to reconsider the asymmetric ΛCCSD[T] model due to both its rigorous theoretical foundation and its performance, which is shown to be similar to that of the CCSD(T) model for systems at equilibrium geometry and superior to it for distorted systems. In both the calculations at equilibrium and distorted geometries, however, the ΛCCSD[T] and CCSD(T) models are shown to be outperformed by the fourthorder CCSD(T4) model.The Journal of Chemical Physics 01/2015; 142(1):014102. DOI:10.1063/1.4904754 · 3.12 Impact Factor 
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ABSTRACT: The equationofmotion coupled cluster (EOMCC) framework has been used for deriving a novel series of perturbative corrections to the coupled cluster singles and doubles energy that formally converges towards the full configuration interaction energy limit. The series is based on a MollerPlesset partitioning of the Hamiltonian and thus size extensive at any order in the perturbation, thereby remedying the major deficiency inherent to previous perturbation series based on the EOMCC ansatz. (C) 2014 AIP Publishing LLC.The Journal of Chemical Physics 05/2014; 140(17):174114. DOI:10.1063/1.4873138 · 3.12 Impact Factor 
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ABSTRACT: Dalton is a powerful generalpurpose program system for the study of molecular electronic structure at the HartreeFock, KohnSham, multiconfigurational selfconsistentfield, MøllerPlesset, configurationinteraction, and coupledcluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronicstructure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gaugeorigininvariant manner. Frequencydependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one, two, and threephoton processes. Environmental effects may be included using various dielectricmedium and quantummechanics/molecularmechanics models. Large molecules may be studied using linearscaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.Wiley interdisciplinary reviews: Computational Molecular Science 05/2014; 4(3):269284. DOI:10.1002/wcms.1172 · 9.04 Impact Factor 
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ABSTRACT: Using the coupled cluster Lagrangian technique, we have determined perturbative corrections to the coupled cluster singles and doubles (CCSD) energy that converge towards the coupled cluster singles, doubles, and triples (CCSDT) and coupled cluster singles, doubles, triples, and quadruples (CCSDTQ) energies, considering the CCSD state as the unperturbed reference state and the fluctuation potential as the perturbation. Since the Lagrangian technique is utilized, the energy corrections satisfy Wigner's 2n + 1 rule for the cluster amplitudes and the 2n + 2 rule for the Lagrange multipliers. The energy corrections define the CCSD perturbation series, CCSD(Tn) and CCSD(TQn), which are termwise size extensive to any order in the perturbation. A detailed comparison of the CCSD(TQn) series and the CC(2)PT(n) series of Hirata et al. [J. Chem. Phys. 114, 3919 (2001)] has been performed, revealing some deficiencies of the latter related to the target energy of the series and its lack of size extensivity.The Journal of Chemical Physics 02/2014; 140(6):064108. DOI:10.1063/1.4862501 · 3.12 Impact Factor 
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ABSTRACT: A common perception about molecular systems with a nonlocal electronic structure (as manifested by a nonlocal Hartree–Fock (HF) density matrix), such as conjugated πsystems, is that they can only be described in terms of nonlocal molecular orbitals. This view is mostly founded on chemical intuition, and further, this view is strengthened by traditional approaches for obtaining local occupied and virtual orbital spaces, such as the occupied Pipek–Mezey orbitals, and projected atomic orbitals. In this article, we discuss the limitations for localizability of HF orbitals in terms of restrictions posed by the delocalized character of the underlying density matrix for the molecular system and by the orthogonality constraint on the molecular orbitals. We show that the locality of the orbitals, in terms of nonvanishing charge distributions of orbitals centered far apart, is much more strongly affected by the orthogonality constraint than by the physical requirement that the occupied orbitals must represent the electron density. Thus, the freedom of carrying out unitary transformations among the orbitals provides the flexibility to obtain highly local occupied and virtual molecular orbitals, even for molecular systems with a nonlocal density matrix, provided that a proper localization function is used. As an additional consideration, we clear up the common misconception that projected atomic orbitals in general are more local than localized orthogonal virtual orbitals.Theoretical Chemistry Accounts 01/2014; 133(1). DOI:10.1007/s002140131417x · 2.14 Impact Factor 
NonLinear Optical Properties of Matter: From molecules to condensed phases, Edited by Manthos G. Papadopoulos and Andrzej J. Sadlej and Jerzy Leszczynski, 10/2013: pages 5199; Springer., ISBN: 1402048491

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ABSTRACT: The shielding and indirect spinspin coupling tensors are calculated for HCN, HNC, CH3CN, and CH3NC, using large atomic basis sets and multiconfiguration wavefunctions that contain the most important correlation effects. The isotropic and anisotropic components of the tensors are compared with experimental data. Deviations between the calculated and experimental results appear to be largely due to vibrational and solvent effects.Journal of Magnetic Resonance Series A 06/2013; DOI:10.1006/jmra.1995.1128 
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ABSTRACT: Using the threelevel energy optimization procedure combined with a refined version of the leastchange strategy for the orbitalswhere an explicit localization is performed at the valence basis levelit is shown how to more efficiently determine a set of local HartreeFock orbitals. Further, a corevalence separation of the leastchange occupied orbital space is introduced. Numerical results comparing valence basis localized orbitals and canonical molecular orbitals as starting guesses for the full basis localization are presented. The results show that the localization of the occupied orbitals may be performed at a small computational cost if valence basis localized orbitals are used as a starting guess. For the unoccupied space, about half the number of iterations are required if valence localized orbitals are used as a starting guess compared to a canonical set of unoccupied HartreeFock orbitals. Different local minima may be obtained when different starting guesses are used. However, the different minima all correspond to orbitals with approximately the same locality. © 2013 Wiley Periodicals, Inc.Journal of Computational Chemistry 06/2013; 34(15). DOI:10.1002/jcc.23256 · 3.60 Impact Factor 
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ABSTRACT: For large molecular systems conventional implementations of second order Møller–Plesset (MP2) theory encounter a scaling wall, both memory and timewise. We describe how this scaling wall can be removed. We present a massively parallel algorithm for calculating MP2 energies and densities using the divide–expand–consolidate scheme where a calculation on a large system is divided into many small fragment calculations employing local orbital spaces. The resulting algorithm is linearscaling with system size, exhibits near perfect parallel scalability, removes memory bottlenecks and does not involve any I/O. The algorithm employs three levels of parallelisation combined via a dynamic job distribution scheme. Results on two molecular systems containing 528 and 1056 atoms (4278 and 8556 basis functions) using 47,120 and 94,240 cores are presented. The results demonstrate the scalability of the algorithm both with respect to the number of cores and with respect to system size. The presented algorithm is thus highly suited for large super computer architectures and allows MP2 calculations on large molecular systems to be carried out within a few hours – for example, the correlated calculation on the molecular system containing 1056 atoms took 2.37 hours using 94240 cores.Molecular Physics 04/2013; 111(911). DOI:10.1080/00268976.2013.783941 · 1.64 Impact Factor 
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ABSTRACT: Recent advances in orbital localization algorithms are used to minimize the PipekMezey localization function for both occupied and virtual HartreeFock orbitals. Virtual PipekMezey orbitals for large molecular systems have previously not been considered in the literature. For this work, the PipekMezey (PM) localization function is implemented for both the Mulliken and a Löwdin population analysis. The results show that the standard PM localization function (using either Mulliken or Löwdin population analyses) may yield local occupied orbitals, although for some systems the occupied orbitals are only semilocal as compared to stateoftheart localized occupied orbitals. For the virtual orbitals, a Löwdin population analysis shows improvement in locality compared to a Mulliken population analysis, but for both Mulliken and Löwdin population analyses, the virtual orbitals are seen to be considerably less local compared to stateoftheart localized orbitals. © 2013 Wiley Periodicals, Inc.Journal of Computational Chemistry 04/2013; DOI:10.1002/jcc.23281 · 3.60 Impact Factor 
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ABSTRACT: DivideExpandConsolidate (DEC) is a local correlation method where the inherent locality of the electron correlation problem is used to express the correlated calculation on a large molecular system in terms of small independent fragment calculations employing small subsets of local HF orbitals. A crucial feature of the DEC scheme is that the sizes of the local orbital spaces are determined in a black box manner during the calculation. In this way it is ensured that the correlation energy has been determined to a predefined precision compared to a conventional calculation. In the present work we apply the DEC scheme to calculate the correlation energy as well as the electron density matrix for the insulin molecule using second order MøllerPlesset (MP2) theory. This is the first DEC calculation on a molecular system which is too large to be treated using a conventional MP2 implementation. The fragmentation errors for the insulin DEC calculation are carefully analyzed using internal consistency checks. It is demonstrated that sizeintensive properties are determined to the same precision for small and large molecules. For example, the percentage of correlation energy recovered and the error per electron in the correlated density matrix depend only on the predefined precision and not on the molecular size.Physical Chemistry Chemical Physics 10/2012; 14(45). DOI:10.1039/c2cp41958k · 4.20 Impact Factor 
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ABSTRACT: We demonstrate that the divideexpandconsolidate (DEC) scheme – which has previously been used to determine the secondorder Møller–Plesset (MP2) correlation energy – can be applied to evaluate the MP2 molecular gradient in a linearscaling and embarrassingly parallel manner using a set of local Hartree–Fock orbitals. All manipulations of fourindex quantities (describing electron correlation effects) are carried out using small local orbital fragment spaces, whereas twoindex quantities are treated for the full molecular system. The sizes of the orbital fragment spaces are determined in a blackbox manner to ensure that the error in the DECMP2 correlation energy compared to a standard MP2 calculation is proportional to a single input threshold denoted the fragment optimization threshold (FOT). The FOT also implicitly controls the error in the DECMP2 molecular gradient as substantiated by a theoretical analysis and numerical results. The development of the DECMP2 molecular gradient is the initial step towards calculating higher order energy derivatives for large molecular systems using the DEC framework, both at the MP2 level of theory and for more accurate coupledcluster methods.The Journal of Chemical Physics 09/2012; 137(11). DOI:10.1063/1.4752432 · 3.12 Impact Factor 
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ABSTRACT: The trust region method has been applied to the minimization of localization functions, and it is shown that both local occupied and local virtual Hartree–Fock (HF) orbitals can be obtained. Because step sizes are size extensive in the trust region method, large steps may be required when the method is applied to large molecular systems. For an exponential parametrization of the localization function only small steps are allowed, and the standard trust radius update therefore has been replaced by a scheme where the direction of the step is determined using a conservative estimate of the trust radius and the length of the step is determined from a line search along the obtained direction. Numerical results for large molecular systems have shown that large steps can then safely be taken, and a robust and nearly monotonic convergence is obtained.Journal of Chemical Theory and Computation 08/2012; 8(9):3137–3146. DOI:10.1021/ct300473g · 5.31 Impact Factor 
Chemical Reviews 01/2012; 112(1):543631. DOI:10.1021/cr2002239 · 45.66 Impact Factor

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ABSTRACT: Previously, we have introduced the linear scaling coupled cluster (CC) divideexpandconsolidate (DEC) method, using an occupied space partitioning of the standard correlation energy. In this article, we show that the correlation energy may alternatively be expressed using a virtual space partitioning, and that the Lagrangian correlation energy may be partitioned using elements from both the occupied and virtual partitioning schemes. The partitionings of the correlation energy leads to atomic site and pair interaction energies which are termwise invariant with respect to an orthogonal transformation among the occupied or the virtual orbitals. Evaluating the atomic site and pair interaction energies using local orbitals leads to a linear scaling algorithm and a distinction between Coulomb hole and dispersion energy contributions to the correlation energy. Further, a detailed error analysis is performed illustrating the error control imposed on all components of the energy by the chosen energy threshold. This error control is ultimately used to show how to reduce the computational cost for evaluating dispersion energy contributions in DEC.The Journal of Chemical Physics 01/2012; 136(1):014105. DOI:10.1063/1.3667266 · 3.12 Impact Factor 
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ABSTRACT: We apply damped response theory to the phenomenon of magnetic circular dichroism (MCD), and we investigate how the numerical instability associated with the simulation of the MCD spectrum from individually calculated A and B terms for close lying states can be remedied by the use of damped response theory. We also present a method for calculating the Faraday A term, formulated as a double residue of the quadratic response function. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3607991]The Journal of Chemical Physics 07/2011; 135(2):024112. DOI:10.1063/1.3607991 · 3.12 Impact Factor 
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ABSTRACT: Damped response theory is applied to the calculation of twophoton absorption (TPA) spectra, which are determined directly, at each frequency, from a modified damped cubic response function. The TPA spectrum may therefore be evaluated for selected frequency ranges, making the damped TPA approach attractive for calculations on large molecules with a high density of states, where the calculation of TPA using standard theory is more problematic. Damped response theory can also be applied to the case of intermediate state resonances, where the standard TPA expression is divergent. Both exact damped response theory and its application within density functional theory are discussed. The latter is implemented using an atomicorbital based density matrix formulation, which makes the approach especially suitable for studies on large systems. A test preliminary study is presented for the TPA spectrum of R(+)1,1'bi(2naphtol).The Journal of Chemical Physics 06/2011; 134(21):214104. DOI:10.1063/1.3595280 · 3.12 Impact Factor
Publication Stats
13k  Citations  
805.50  Total Impact Points  
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Institutions

1454–2015

Aarhus University
 Department of Chemistry
Aarhus, Central Jutland, Denmark


1994–2004

University of Oslo
 Department of Chemistry
Kristiania (historical), Oslo County, Norway


1995

The Police Academy of the Czech Republic in Prague
Praha, Praha, Czech Republic 
University of Santiago de Compostela
 Department of Physical Chemistry
Santiago, Galicia, Spain


1989–1994

Uppsala University
Uppsala, Uppsala, Sweden


1993

Linköping University
 Department of Physics, Chemistry and Biology (IFM)
Linköping, Östergötland, Sweden


1992

Odense University Hospital
Odense, South Denmark, Denmark


1989–1991

Lund University
 Department of Theoretical Chemistry
Lund, Skåne, Sweden


1987

University of Minnesota Duluth
 Department of Chemistry and Biochemistry
Duluth, Minnesota, United States


1979

Texas A&M University
 Department of Chemistry
College Station, TX, United States


1975

University of Utah
 Department of Chemistry
Salt Lake City, Utah, United States 
CUNY Graduate Center
New York City, New York, United States
