A density-fitting implementation of the density-based basis-set correction method.

This work reports an efficient density-fitting implementation of the density-based basis-set correction (DBBSC) method in the MOLPRO software. This method consists in correcting the energy calculated by a wave-function method with a given basis set by an adapted basis-set correction density functional incorporating the short-range electron correlation effects missing in the basis set, resulting in an accelerated convergence to the complete-basis-set limit. Different basis-set correction density-functional approximations are explored and the complementary-auxiliary-basis-set single-excitation correction is added.

Thermochemical evaluation of adaptive and fixed density functional theory quadrature schemes.

A systematic study is made of the accuracy and efficiency of a number of existing quadrature schemes for molecular Kohn-Sham Density-Functional Theory (DFT) using 408 molecules and 254 chemical reactions. Included are the fixed SG-x (x = 0-3) grids of Gill et al., Dasgupta, and Herbert, the 3-zone grids of Treutler and Ahlrichs, a fixed five-zone grid implemented in Molpro, and a new adaptive grid scheme.

The Molpro quantum chemistry package.

Molpro is a general purpose quantum chemistry software package with a long development history. It was originally focused on accurate wavefunction calculations for small molecules but now has many additional distinctive capabilities that include, inter alia, local correlation approximations combined with explicit correlation, highly efficient implementations of single-reference correlation methods, robust and efficient multireference methods for large molecules, projection embedding, and anharmonic vibrational spectra. In addition to conventional input-file specification of calculations, Molpro calculations can now be specified and analyzed via a new graphical user interface and through a Python framework.

Intermolecular interaction energies from fourth order many-body perturbation theory. Impact of individual electron correlation contributions.

The performance of Møller-Plesset perturbation theory methods for describing intermolecular interaction energies has been investigated with the focus on illuminating the impact of individual electron correlation energy contributions in fourth order. It is shown that a physically meaningful decomposition of the fourth order correlation energy can be obtained by grouping individual correlation energy terms that share the same diagrammatic loop structure. This decomposition of the fourth order singles (S), doubles (D), triples (T), and quadruples (Q) terms revealed that individual terms from each excitation class can have a huge impact on the energy that is much larger than the total fourth order correlation contribution.

Correction to: The Feynman dispersion correction for MNDO extended to F, Cl, Br and I.

A small coding error in the development version of EMPIRE led to some inconsistencies in the above article. They are corrected in this erratum.

The Feynman dispersion correction for MNDO extended to F, Cl, Br and I.

The recently introduced "Feynman" dispersion correction for MNDO (MNDO-F) has been extended to include the elements fluorine, chlorine, bromine and iodine and the original parameterization for hydrogen, carbon, nitrogen and oxygen improved by allowing individual damping radii for the elements. MNDO-F gives a root-mean-square deviation to reference interaction energies of 0.35 kcal mol for the complete parameterization dataset of H, C, N, O, F, Cl, Br and I containing compounds.

Study of the Wilcox torsion balance in solution for a Tröger's base derivative with hexyl-and heptyl substituents using a combined molecular mechanics and quantum chemistry approach.

The folding equilibrium of the Wilcox torsion balance in solution has been studied using a molecular mechanics method for sampling the conformational space and semi-empirical and density-functional quantum chemistry methods for characterizing the relative stabilities of various solute-solvent clusters extracted with the aid of the MD-quench technique from the different simulations that were performed. The role of the solvent environment has been analyzed by choosing four solvents of different polarities, namely water, acetone, tetrachloromethane, and n-hexane. In all cases, it is found that the attractive intramolecular interactions in folded conformations are strongly compensated by the increase of the solute-solvent interaction energies when the molecule unfolds.

The coulombic σ-hole model describes bonding in CXIY complexes completely.

Contrary to recent reports, the σ-hole interaction energies of complexes between the carbon tetrahalides CX3I (X = F, Cl, Br, I) and halide anions Y- (Y = F, Cl, Br, I) are described very well by the simple Coulombic σ-hole concept if it is applied properly. There is no need to invoke charge transfer, which in any case is not uniquely distinguishable from polarization.

Geometry optimisations with a nonlocal density-functional theory method based on a double Hirshfeld partitioning.

Energy gradients have been derived for the nonlocal density-functional theory (NLDFT) method from Heßelmann [J. Chem. Theory Comput.

Correlation effects and many-body interactions in water clusters.

The quantum-chemical description of the interactions in water clusters is an essential basis for deriving accurate and physically sound models of the interaction potential for water to be used in molecular simulations. In particular, the role of many-body interactions beyond the two-body interactions, which are often not explicitly taken into account by empirical force fields, can be accurately described by quantum chemistry methods on an adequate level, e.g.

DFT-SAPT Intermolecular Interaction Energies Employing Exact-Exchange Kohn-Sham Response Methods.

Intermolecular interaction energies have been calculated by symmetry-adapted perturbation theory based on density functional theory monomer properties (DFT-SAPT) employing response functions from time-dependent exact-exchange (TDEXX) kernels. Combined with a new asymptotic correction scheme for the exchange-correlation (xc) potentials of the monomers, which is similar in its performance to standard asymptotic correction methods, it is shown that this DFT-SAPT[TDEXX] method delivers highly accurate intermolecular interaction energies for the S22, S66, and IonHB benchmark databases by Hobza et al. A corresponding DFT-SAPT approach employing the adiabatic TDEXX kernel in the response calculations has also been tested.

Low scaling random-phase approximation electron correlation method including exchange interactions using localised orbitals.

A random-phase approximation electron correlation method including exchange interactions has been developed which reduces the scaling behaviour of the standard approach by two to four orders of magnitude, effectively leading to a linear scaling performance if the local structures of the underlying quantities are fully exploited in the calculations. This has been achieved by a transformation of the integrals and amplitudes from the canonical orbital basis into a local orbital basis and a subsequent dyadic screening approach. The performance of the method is demonstrated for a range of tripeptide molecules as well as for two conformers of the polyglycine molecule using up to 40 glycine units.

Trifluoromethyl: An Amphiphilic Noncovalent Bonding Partner.

The traditional "F " picture of fluorine suggests that it can only interact with electrophilic centers such as backbone-carbonyl carbon atoms or hydrogen-bond donors in proteins. We show that this view, which neglects polarization, is incomplete and the trifluoromethyl groups can act both as electrophiles and nucleophiles to form noncovalent interactions. The underlying polarization mechanism is based on the anomeric effect and is only fully operative if the geometry is allowed to relax.

Accurate Intermolecular Potential for the C Dimer: The Performance of Different Levels of Quantum Theory.

The self-assembly of molecular building blocks is a promising route to low-cost nanoelectronic devices. It would be very appealing to use computer-aided design to identify suitable molecules. However, molecular self-assembly is guided by weak interactions, such as dispersion, which have long been notoriously difficult to describe with quantum chemical methods.

On the Stability of Cyclophane Derivates Using a Molecular Fragmentation Method.

A molecular fragmentation method is used to study the stability of cyclophane derivates by decomposing the molecular energy into the molecular strain and intramolecular interaction energies. The molecular strain energies obtained by utilising the fragmentation method are in good agreement with existing experimental data. The intramolecular interaction energies calculated as the difference between the supermolecular energy and the bonded fragment energies are repulsive in the cyclophanes studied.

Local Molecular Orbitals from a Projection onto Localized Centers.

A localization method for molecular orbitals is presented which exploits the locality of the eigenfunctions associated with the largest eigenvalues of the matrix representation of spatially localized functions. Local molecular orbitals are obtained by a projection of the canonical orbitals onto the set of the eigenvectors which correspond to the largest eigenvalues of these matrices. Two different types of spatially localized functions were chosen in this work, a two-parameter smooth-step-type function and the weight functions determined by a Hirshfeld partitioning of the molecular volume.

Molecular energies from an incremental fragmentation method.

The systematic molecular fragmentation method by Collins and Deev [J. Chem. Phys.

Molecular Excitation Energies from Time-Dependent Density Functional Theory Employing Random-Phase Approximation Hessians with Exact Exchange.

Molecular excitation energies have been calculated with time-dependent density-functional theory (TDDFT) using random-phase approximation Hessians augmented with exact exchange contributions in various orders. It has been observed that this approach yields fairly accurate local valence excitations if combined with accurate asymptotically corrected exchange-correlation potentials used in the ground-state Kohn-Sham calculations. The inclusion of long-range particle-particle with hole-hole interactions in the kernel leads to errors of 0.

Polarisabilities of long conjugated chain molecules with density functional response methods: The role of coupled and uncoupled response.

The longitudinal component of the dipole-dipole polarisability of polyacetylene molecules containing 4 to 20 carbon atoms has been calculated with density-functional theory (DFT) response methods. In order to analyse the effect of the uncoupled and coupled contributions to the response matrix, a number of different sets of orbitals were combined with different approximations for the Hessian matrix. This revealed a surprising result: a qualitatively correct increase of the polarisability with the chain length can already be reproduced on the uncoupled level if the response matrix is constructed from Hartree-Fock (HF) or exact-exchange (EXX) DFT orbitals.

Intermolecular symmetry-adapted perturbation theory study of large organic complexes.

Binding energies for the complexes of the S12L database by Grimme [Chem. Eur. J.

On the Short-Range Behavior of Correlated Pair Functions from the Adiabatic-Connection Fluctuation-Dissipation Theorem of Density-Functional Theory.

The short-range behavior of correlated pair functions from the adiabatic-connection fluctuation-dissipation theorem (ACFD) of density functional theory (DFT) employing local exchange-correlation kernels has been analyzed. It has been found that for large basis sets the pair function exhibits unphysical humps for small interelectronic distances if the adiabatic local density approximation kernel is used in the ACFD scheme (this method is termed ACFD/ALDA in this work). However, up to basis set sizes of quadruple-ζ type quality, the correlated pair function of ACFD/ALDA behaves physically correct and the method yields reasonable results for atomization energies, ionization potentials, and intermolecular interaction energies.

Efficient self-consistent treatment of electron correlation within the random phase approximation.

A self-consistent Kohn-Sham (KS) method is presented that treats correlation on the basis of the adiabatic-connection dissipation-fluctuation theorem employing the direct random phase approximation (dRPA), i.e., taking into account only the Coulomb kernel while neglecting the exchange-correlation kernel in the calculation of the Kohn-Sham correlation energy and potential.

Directional Noncovalent Interactions: Repulsion and Dispersion.

The interaction energies between an argon atom and the dihalogens Br2, BrCl, and BrF have been investigated using frozen core CCSD(T)(fc)/aug-cc-pVQZ calculations as reference values for other levels of theory. The potential-energy hypersurfaces show two types of minima: (1) collinear with the dihalogen bond and (2) in a bridging position. The former represent the most stable minima for these systems, and their binding energies decrease in the order Br > Cl > F.

Assessment of a Nonlocal Correction Scheme to Semilocal Density Functional Theory Methods.

A nonlocal correction method to (semi)local density-functional theory (DFT) methods is derived that is based on a partitioning of the correlation-energy density into atom-atom contributions. Nonlocal interaction contributions, which are absent in standard DFT methods, are introduced in this method by using atomic weight functions that do not vanish exponentially as the atomic densities but with the inverse sixth power of the atomic distances. The parameters contained in these weight functions were fitted to reproduce intermolecular interaction energies for a range a small dimer systems.