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ABSTRACT: The Joule-Thomson coefficient μ(P, T) is computed from the virial equation of state up to seventh-order for argon obtained from accurate ab initio data. Higher-order corrections become increasingly more important to fit the low-temperature and low-pressure regime and to avoid the early onset of divergence in the Joule-Thomson inversion curve. Good agreement with experiment is obtained for temperatures T > 250 K. The results also illustrate the limitations of the virial equation in regions close to the critical temperature.
The Journal of chemical physics 02/2013; 138(7):071105. · 3.09 Impact Factor
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ABSTRACT: Second, third, and fourth thermal and acoustic virial coefficients between 100 and 1000 K are computed for different argon interaction models derived from combinations of accurate two- and three-body potentials. Differences between the various interaction models tested mirror the presumed order in the accuracy of these models, but are not well captured at the level of the lowest-order contributions in the virial expansion: While the second- and third-order virial coefficients are found to be rather insensitive to small variations in the two- and three-body potentials, more pronounced differences in higher-order coefficients are currently of limited use in assessing the accuracy of the interaction potential due to difficulties in the unambiguous experimental determination of these higher-order coefficients. In contrast, pressure-volume and speed-of-sound data--both of which are experimentally known to highest accuracies--are found to be insensitive to small variations in the interaction model. All but the least accurate models reproduce experimental pressure-volume and speed-of-sound data near-quantitatively in regions where the (fourth-order) virial expansions apply. All quantities considered are found to be completely unaffected by a non-vanishing quadruple-dipole four-body potential.
The Journal of chemical physics 08/2012; 137(6):064702. · 3.09 Impact Factor
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ABSTRACT: A simple and efficient internal-coordinate importance sampling protocol for the Monte Carlo computation of (up to fourth-order) virial coefficients ̅B(n) of atomic systems is proposed. The key feature is a multivariate sampling distribution that mimics the product structure of the dominating pairwise-additive parts of the ̅B(n). This scheme is shown to be competitive over routine numerical methods and, as a proof of principle, applied to neon: The second, third, and fourth virial coefficients of neon as well as equation-of-state data are computed from ab initio two- and three-body potentials; four-body contributions are found to be insignificant. Kirkwood-Wigner quantum corrections to first order are found to be crucial to the observed agreement with recent ab initio and experimental reference data sets but are likely inadequate at very low temperatures.
The Journal of chemical physics 07/2012; 137(1):014508. · 3.09 Impact Factor
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ABSTRACT: In a previous paper we pointed out that core contributions to the static magnetizability are non-negligible, and can therefore lead to erroneous results within the pseudopotential approximation [P. Schwerdtfeger, B. Assadollahzadeh, U. Rohrmann, R. Schäfer, and J. R. Cheeseman, J. Chem. Phys. 134, 204102 (2011)]. In a recent paper van Wüllen showed that additional terms arising from the gauge-invariant condition for the semi-local part of a pseudopotential operator can lead to non-negligible contributions to the magnetizability tensor, which are sensitive to the gauge origin and basis set chosen [C. van Wüllen, J. Chem. Phys. 136, 114110 (2012)]. These terms were neglected in previous calculations as they were assumed to be small. In this paper we analyze the importance of the gauge-dependent semi-local pseudopotential correction term in detail for AuF and clusters of Sn showing that it leads indeed to very large corrections to the paramagnetic term for large-core pseudopotentials. Without this correction the results become very sensitive to the basis set applied. This now resolves some of the unusual large paramagnetic contributions reported before for both AuF and Sn(2).
The Journal of chemical physics 07/2012; 137(1):014107. · 3.09 Impact Factor
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ABSTRACT: Potential energy curves of the electronic ground states of the group 12 dimers Zn2 and Cd2 were computed at the CCSD(T) level of theory, including full triple corrections
\Updelta\UpdeltaT in the coupled-cluster procedure, and spin-orbit (SO) contributions from four-component coupled-cluster calculations, extrapolated
to the complete basis set (CBS) limit. For Hg2, the potential energy curve published recently (Pahl et al. in J Chem Phys 132:114301, 2010] is complemented in this work by non-relativistic calculations to quantify and discuss relativistic effects. We obtain very
accurate fits of our CBS/CCSD(T) and CBS/CCSD(T)+
\Updelta\UpdeltaT data points to an analytically simple and computationally efficient extended Lennard Jones form. For the CBS/CCSD(T)+
\Updelta\UpdeltaT+SO curves, we obtain dissociation energies of D
e=226cm−1 and D
e=319cm−1 for Zn2 and Cd2 respectively, in very good agreement with recent theoretical calculations and experimental data. We also present equilibrium
distances and rotational and vibrational spectroscopic constants to compare with available theoretical and experimental data.
The results obtained for non-relativistically treated Hg2 continue nicely the trends with increasing atom number preset by Zn2 and Cd2, confirming that indeed, relativistic effects account for the known peculiarities for the mercury dimer.
KeywordsPotential energy curves–Zn dimer–Cd dimer–Hg dimer–Relativistic effects
Theoretical Chemistry Accounts 04/2012; 129(3):651-656. · 2.16 Impact Factor
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ABSTRACT: The possible existence of the gas phase cis- and trans-maleate, i.e. completely deprotonated maleic acid (O2C–CΗ=CΗ–CO2)2–, is investigated by density functional (B3LYP) and ab-initio quantum chemical methods (MP2, CCSD(T)) using large basis sets.
The calculations reveal that only the trans-isomer is Coulomb stable with respect to electron loss. The results are compared to other previously investigated dicarboxylate
dianions of the general form −O2C–R–CO2
− with R=C2, C2X2, C2X4, and
C6X4 (X=H, F). Fluorine substitution on the carbon framework helps to stabilize these doubly charged systems, and we predict
that all of the aromatic fluorine substituted dicarboxylate dianions are Coulomb stable in the gas phase. Only the highest
levels of theory reveal the slight stabilization of both the succinate dianion and the ortho-isomer of the phthalic acid dianion in unprecedented agreement with experiments.
KeywordsDicarboxylate dianions-Density functional theory-Wavefunction-based theory-Coulomb repulsion-Charge conjugation
Theoretical Chemistry Accounts 04/2012; 126(3):129-138. · 2.16 Impact Factor
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ABSTRACT: Multistrand 7-1 helical Au(24), Au(32), and Au(40) structures with three, four, and five gold atoms in the central strand and 21, 28, and 35 gold atoms in the coaxial tube are investigated using relativistic density functional theory. We demonstrate that these helical gold nanorods are stable structures with a rather large HOMO-LUMO gap, a large binding energy per atom, a very large vertical dissociation energy, and an extremely large electron affinity. On the basis of the atomic charges and the nature of the frontier orbitals, they are also expected to have strong selective reactivity toward electrophiles and nucleophiles. Furthermore, we show that these helical Au(n) structures and, in particular, the helical Au(40) structure are competitive energetically and chemically with respect to alternate cage and compact Au(n) structures. We consider two fragmentations of the helical Au(40) structure and perform a density of states analysis to examine both charge transfer and electronic polarization.
Journal of Computational Chemistry 11/2011; 33(3):311-8. · 4.58 Impact Factor
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Peter Schwerdtfeger
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ABSTRACT: A short review is presented on one of the most successful theories for electronic structure calculations, the pseudopotential approximation, originally introduced by Hans G. A. Hellmann in 1934. Recent developments in relativistic quantum theory allow for the accurate adjustment of pseudopotential parameters to valence spectra, producing results for properties of atoms, molecules, and the solid-state in excellent agreement with more accurate all-electron results if a small-core definition is used. Thus the relativistic pseudopotential approximation is now the most widely applied method for systems containing heavy elements.
ChemPhysChem 08/2011; 12(17):3143-55. · 3.41 Impact Factor
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ABSTRACT: How many rare gas atoms can be placed into a fullerene cage until the pressure becomes large enough to break the C(60) framework? The answer given by density functional and ab initio computations is surprising and underlines the high stability of this unique carbon structure.
ChemPhysChem 06/2011; 12(11):2081-4. · 3.41 Impact Factor
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ABSTRACT: The response of the electronic wavefunction to an external electric or magnetic field is widely considered to be a typical valence property and should, therefore, be adequately described by accurately adjusted pseudopotentials, especially if a small-core definition is used within this approximation. In this paper we show for atomic Au and Au(+), as well as for the molecule AuF and tin clusters, that in contrast to the case of the static electric dipole polarizability or the electric dipole moment, core contributions to the static magnetizability are non-negligible, and can therefore lead to erroneous results within the pseudopotential approximation. This error increases with increasing size of the core chosen. For tin clusters, which are of interest in ongoing molecular beam experiments currently carried out by the Darmstadt group, the diamagnetic and paramagnetic isotropic components of the magnetizability tensor almost cancel out and large-core pseudopotentials do not even predict the correct sign for this property due to erroneous results in both the diamagnetic and (more importantly) the paramagnetic terms. Hence, all-electron calculations or pseudopotentials with very small cores are required to adequately predict magnetizabilities for atoms, molecules and the solid state, making it computationally more difficult to obtain this quantity for future investigations in heavy atom containing molecules or clusters. We also demonstrate for this property that all-electron density functional calculations are quite robust and give results close to wavefunction based methods for the atoms and molecules studied here.
The Journal of chemical physics 05/2011; 134(20):204102. · 3.09 Impact Factor
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ABSTRACT: First-principles calculations show that the optical UV absorption onset of solid water is blueshifted with increasing pressure. Across several crystal structures and a wide pressure range, the optical gap increases almost linearly with external pressure, making solid water more transparent. The origin of this unusual effect can be traced back to an increased Stark shift caused by water's electrostatic environment at smaller volumes.
Physical Review Letters 05/2011; 106(18):187403. · 7.37 Impact Factor
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ABSTRACT: We argue that Suhm and Albrecht's key point that the homo/heterochiral dimers do not play any role in the process of self-disproportionation of enantiomers of α-(trifluoromethyl)lactic acid through sublimation might be valid under a particular set of experimental conditions. However, the formation of dimers in the gas-phase cannot be completely ruled out, in particular under the originally reported conditions of evaporative sublimation at ambient pressure and temperature. Molecular dynamic simulations of the sublimation process together with more accurate experimental data are required to fully explain this intriguing phenomenon.
Physical Chemistry Chemical Physics 03/2011; 13(9):4161-2. · 3.57 Impact Factor
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ABSTRACT: There are many different hypotheses on the origin of biomolecular homochirality. One possible scenario concerns the enantiomeric enrichment of a nearly racemic solid via self-disproportionation of enantiomers. In particular, in a recent paper Soloshonok and co-workers showed a first example of optical self-purification of α-(trifluoromethyl)lactic acid by sublimation [V. A. Soloshonok et al., J. Am. Chem. Soc. 2007, 129, 12112]. Here we present detailed theoretical studies of α-(trifluoromethyl)lactic acid in the solid state as well as in the gas-phase dimeric form. The calculations of energy differences between dimers show that in the solid state the enantiomeric pure compound is energetically preferred, while in the gas phase the equilibrium shifts towards the racemic mixture although thermodynamic corrections cannot be neglected, thus providing a detailed microscopic explanation for the enantio-purification process for the first time.
Physical Chemistry Chemical Physics 01/2011; 13(3):811-7. · 3.57 Impact Factor
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ABSTRACT: In order to guide the experimental search for parity violation in molecular systems, in part motivated by the possible link to biomolecular homochirality, we present a detailed analysis in a relativistic framework of the mechanism behind the tiny energy difference between enantiomers induced by the weak force. A decomposition of the molecular expectation value into atomic contributions reveals that the effect can be thought of as arising from a specific mixing of valence s(1/2) and p(1/2) orbitals on a single center induced by a chiral molecular field. The intra-atomic nature of the effect is further illustrated by visualization of the electron chirality density and suggests that a simple model for parity violation in molecules may be constructed by combining pre-calculated atomic quantities with simple bonding models. A 2-component relativistic computational procedure is proposed which bridges the relativistic and non-relativistic approaches to the calculation of parity violation in chiral molecules and allows us to explore the single-center theorem in a variational setting.
Physical Chemistry Chemical Physics 01/2011; 13(3):864-76. · 3.57 Impact Factor
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ABSTRACT: There are many different hypotheses on the origin of biomolecular homochirality. One possible scenario concerns the enantiomeric enrichment of a nearly racemic solid via self-disproportionation of enantiomers. In particular, in a recent paper Soloshonok and co-workers showed a first example of optical self-purification of alpha-(trifluoromethyl)lactic acid by sublimation [V. A. Soloshonok et al., J. Am. Chem. Soc. 2007, 129, 12112]. Here we present detailed theoretical studies of alpha-(trifluoromethyl)lactic acid in the solid state as well as in the gas-phase dimeric form. The calculations of energy differences between dimers show that in the solid state the enantiomeric pure compound is energetically preferred, while in the gas phase the equilibrium shifts towards the racemic mixture although thermodynamic corrections cannot be neglected, thus providing a detailed microscopic explanation for the enantio-purification process for the first time.
Physical Chemistry Chemical Physics 01/2011; 13(3):811-7. · 3.57 Impact Factor
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ABSTRACT: How many rare gas atoms can be placed into a fullerene cage until the pressure becomes large enough to break the C(60) framework? The answer given by density functional and ab initio computations is surprising and underlines the high stability of this unique carbon structure.
ChemPhysChem 01/2011; 12(11):2081-2084. · 3.41 Impact Factor
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Benoît Darquié,
Clara Stoeffler,
Alexander Shelkovnikov,
Christophe Daussy,
Anne Amy-Klein,
Christian Chardonnet,
Samia Zrig,
Laure Guy,
Jeanne Crassous,
Pascale Soulard,
Pierre Asselin,
Thérèse R Huet, Peter Schwerdtfeger,
Radovan Bast,
Trond Saue
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ABSTRACT: Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.
Chirality 11/2010; 22(10):870-84. · 2.35 Impact Factor
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Peter Schwerdtfeger
10/2010: pages 201 - 221; , ISBN: 9783527633272
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ABSTRACT: Spin-orbit effects approximately scale like Z 2 and therefore become very important in the bonding of the heavier p-group elements in the periodic table. Here we show by first-principles density-functional calculations that such effects substantially lower the cohesive energy for solid lead and Uuq ununquadium, eka-lead, nuclear charge 114, by 2.5 eV/atom for the latter and causing a structural change from face-centered cubic at the scalar relativistic to hexagonal close packed at the spin-orbit coupled level of theory. This implies that unlike lead cohesive energy E coh = 2.02 eV/ atom, Uuq is weakly bound E coh = 0.5 eV/ atom, and even less so than solid mercury E coh = 0.7 eV/ atom, underpinning the original hypothesis by Pitzer in 1975 K. Pitzer, J. Chem. Phys. 63, 1033 1975 that spin-orbit effects lead to chemical inertness of Uuq.
Physical Review B 10/2010; 82:155116. · 3.69 Impact Factor
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ABSTRACT: We report the synthesis and resolution of a series of new chiral "3 + 1" oxorhenium(V) complexes, designed for high-resolution laser spectroscopy experiments probing molecular parity-violation (PV) effects in the Re=O stretching mode frequency. These complexes display a particularly simple chemical structure, with the rhenium atom as the stereogenic center, and show large PV energy differences according to our calculations. They were obtained in the racemic and enantioenriched forms, in the latter case by using either semi-preparative chiral HPLC resolution or enantioselective synthesis. The vibrational transition frequency differences between the enantiomeric pairs due to PV have been calculated with two- and four-component relativistic Hamiltonians using Hartree-Fock (HF) and density functional theory (DFT). For three complexes, including one synthesized in enantioenriched form, our HF calculations predict frequency differences above the present resolution limit of 1 Hz. These results confirm the order of magnitude for the calculated HF PV vibrational frequency differences reported earlier for this class of compounds [P. Schwerdtfeger and R. Bast, J. Am. Chem. Soc., 2004, 126, 1652]. However, at the DFT level the PV vibrational frequency differences are in some cases reduced by an order of magnitude, but are still within the sensitivity of 0.01 Hz, which is the anticipated sensitivity in a new proposed experiment. We therefore believe that the present study represents an important step towards the experimental observation of PV in molecular systems, and emphasizes the extreme sensitivity of the PV vibrational frequency difference to the chemical environment around the rhenium center.
Physical Chemistry Chemical Physics 08/2010; 12(31):8792-803. · 3.57 Impact Factor
