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Beyond the random-phase approximation for the electron correlation energy: The importance of single excitations ; Tkatchenko, Alexandre ; et al in Physical Review Letters (2011), 106(15), The random-phase approximation (RPA) for the electron correlation energy, combined with the exact-exchange (EX) energy, represents the state-of-the-art exchange-correlation functional within density ... [more ▼] The random-phase approximation (RPA) for the electron correlation energy, combined with the exact-exchange (EX) energy, represents the state-of-the-art exchange-correlation functional within density-functional theory. However, the standard RPA practice-evaluating both the EX and the RPA correlation energies using Kohn-Sham (KS) orbitals from local or semilocal exchange-correlation functionals-leads to a systematic underbinding of molecules and solids. Here we demonstrate that this behavior can be corrected by adding a "single excitation" contribution, so far not included in the standard RPA scheme. A similar improvement can also be achieved by replacing the non-self-consistent EX total energy by the corresponding self-consistent Hartree-Fock total energy, while retaining the RPA correlation energy evaluated using KS orbitals. Both schemes achieve chemical accuracy for a standard benchmark set of noncovalent intermolecular interactions. © 2011 American Physical Society. [less ▲] Detailed reference viewed: 172 (0 UL)Unraveling the stability of polypeptide helices: Critical role of van der Waals interactions Tkatchenko, Alexandre ; ; et al in Physical Review Letters (2011), 106(11), Folding and unfolding processes are important for the functional capability of polypeptides and proteins. In contrast with a physiological environment (solvated or condensed phases), an in vacuo study ... [more ▼] Folding and unfolding processes are important for the functional capability of polypeptides and proteins. In contrast with a physiological environment (solvated or condensed phases), an in vacuo study provides well-defined "clean room" conditions to analyze the intramolecular interactions that largely control the structure, stability, and folding or unfolding dynamics. Here we show that a proper consideration of van der Waals (vdW) dispersion forces in density-functional theory (DFT) is essential, and a recently developed DFT+vdW approach enables long time-scale ab initio molecular dynamics simulations at an accuracy close to "gold standard" quantum-chemical calculations. The results show that the inclusion of vdW interactions qualitatively changes the conformational landscape of alanine polypeptides, and greatly enhances the thermal stability of helical structures, in agreement with gas-phase experiments. © 2011 American Physical Society. [less ▲] Detailed reference viewed: 171 (0 UL)Van der Waals interactions in ionic and semiconductor solids ; Tkatchenko, Alexandre ; et al in Physical Review Letters (2011), 107(24), Van der Waals (vdW) energy corrected density-functional theory is applied to study the cohesive properties of ionic and semiconductor solids (C, Si, Ge, GaAs, NaCl, and MgO). The required polarizability ... [more ▼] Van der Waals (vdW) energy corrected density-functional theory is applied to study the cohesive properties of ionic and semiconductor solids (C, Si, Ge, GaAs, NaCl, and MgO). The required polarizability and dispersion coefficients are calculated using the dielectric function obtained from time-dependent density-functional theory. Coefficients for "atoms in the solid" are then calculated from the Hirshfeld partitioning of the electron density. It is shown that the Clausius-Mossotti equation that relates the polarizability and the dielectric function is accurate even for covalently-bonded semiconductors. We find an overall improvement in the cohesive properties of Si, Ge, GaAs, NaCl, and MgO, when vdW interactions are included on top of the Perdew-Burke- Ernzerhof or Heyd-Scuseria-Ernzerhof functionals. The relevance of our findings for other solids is discussed. © 2011 American Physical Society. [less ▲] Detailed reference viewed: 196 (3 UL)Hydrogen bonds and van der Waals forces in ice at ambient and high pressures ; ; et al in Physical Review Letters (2011), 107(18), The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure ... [more ▼] The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated. © 2011 American Physical Society. [less ▲] Detailed reference viewed: 164 (0 UL)Describing both dispersion interactions and electronic structure using density functional theory: The case of metal-phthalocyanine dimers ; Tkatchenko, Alexandre ; et al in Journal of Chemical Theory and Computation (2010), 6(1), 81-90 Noncovalent interactions, of which London dispersion is an important special case, are essential to many fields of chemistry. However, treatment of London dispersion is inherently outside the reach of ... [more ▼] Noncovalent interactions, of which London dispersion is an important special case, are essential to many fields of chemistry. However, treatment of London dispersion is inherently outside the reach of (semi)local approximations to the exchange-correlation functional as well as of conventional hybrid density functionals based on semilocal correlation. Here, we offer an approach that provides a treatment of both dispersive interactions and the electronic structure within a computationally tractable scheme. The approach is based on adding the leading interatomic London dispersion term via pairwise ion-ion interactions to a suitably chosen nonempirical hybrid functional, with the dispersion coefficients and van der Waals radii determined from first-principles using the recently proposed "TS-vdW" scheme (Tkatchenko, A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005). This is demonstrated via the important special case of weakly bound metal-phthalocyanine dimers. The performance of our approach is additionally compared to that of the semiempirical M06 functional. We find that both the PBE-hybrid+vdW functional and the M06 functional predict the electronic structure and the equilibrium geometry well, but with significant differences in the binding energy and in their asymptotic behavior. Copyright © 2010 American Chemical Society. [less ▲] Detailed reference viewed: 146 (0 UL)Accurate molecular van der Waals interactions from ground-state electron density and free-atom reference data Tkatchenko, Alexandre ; in Physical Review Letters (2009), 102(7), We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of ... [more ▼] We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C6 coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C6 coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchange-correlation functional. We show that the effective atomic C6 coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C6R-6 correction method for density-functional theory calculations. © 2009 The American Physical Society. [less ▲] Detailed reference viewed: 214 (4 UL)On the accuracy of density-functional theory exchange-correlation functionals for H bonds in small water clusters. II. the water hexamer and van der Waals interactions ; ; et al in Journal of Chemical Physics (2008), 129(19), Second order Møller-Plesset perturbation theory at the complete basis set limit and diffusion quantum Monte Carlo are used to examine several low energy isomers of the water hexamer. Both approaches ... [more ▼] Second order Møller-Plesset perturbation theory at the complete basis set limit and diffusion quantum Monte Carlo are used to examine several low energy isomers of the water hexamer. Both approaches predict the so-called prism to be the lowest energy isomer, followed by cage, book, and cyclic isomers. The energies of the four isomers are very similar, all being within 10-15 meV/ H2 O. These reference data are then used to evaluate the performance of several density-functional theory exchange-correlation (xc) functionals. A subset of the xc functionals tested for smaller water clusters [I. Santra, J. Chem. Phys. 127, 184104 (2007)] has been considered. While certain functionals do a reasonable job at predicting the absolute dissociation energies of the various isomers (coming within 10-20 meV/ H2 O), none predict the correct energetic ordering of the four isomers nor does any predict the correct low total energy isomer. All xc functionals tested either predict the book or cyclic isomers to have the largest dissociation energies. A many-body decomposition of the total interaction energies within the hexamers leads to the conclusion that the failure lies in the poor description of van der Waals (dispersion) forces in the xc functionals considered. It is shown that the addition of an empirical pairwise (attractive) C6 R-6 correction to certain functionals allows for an improved energetic ordering of the hexamers. The relevance of these results to density-functional simulations of liquid water is also briefly discussed. © 2008 American Institute of Physics. [less ▲] Detailed reference viewed: 159 (0 UL) |
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