Peter Reinhardt

Towards a force field based on density fitting

Total intermolecular interaction energies are determined with a first version of the Gaussian electrostatic model (GEM-0), a force field based on a density fitting approach using s-type Gaussian functions. The total interaction energy is computed in the spirit of the sum of interacting fragment ab initio (SIBFA) force field by separately evaluating each one of its components: electrostatic (Coulomb), exchange repulsion, polarization, and charge transfer intermolecular interaction energies, in order to reproduce reference constrained space orbital variation (CSOV) energy decomposition calculations at the B3LYP/aug-cc-pVTZ level. The use of an auxiliary basis set restricted to spherical Gaussian functions facilitates the rotation of the fitted densities of rigid fragments and enables a fast and accurate density fitting evaluation of Coulomb and exchange-repulsion energy, the latter using the overlap model introduced by Wheatley and Price [Mol. Phys. 69, 50718 (1990)]. The SIBFA energy scheme for polarization and charge transfer has been implemented using the electric fields and electrostatic potentials generated by the fitted densities. GEM-0 has been tested on ten stationary points of the water dimer potential energy surface and on three water clusters (n = 16,20,64). The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature. Preliminary results for Coulomb and exchange-repulsion energies of metal cation complexes and coupled cluster singles doubles electron densities are discussed.

Fragment-Localized Kohn−Sham Orbitals via a Singles Configuration-Interaction Procedure and Application to Local Properties and Intermolecular Energy Decomposition Analysis†

As for generating localized Hartree-Fock orbitals, we propose a potentially linear-scaling singles-CI scheme to construct fragment-localized density functional theory (DFT) orbitals for molecular systems as water clusters. Due to the use of a deformation step instead of a localization step, the influence of the environment on each separate molecule can be studied in detail. The generated orbital set for the whole molecular system is strictly equivalent to a set of canonical orbitals and is a subsequent energy decomposition of intermolecular interactions into electrostatic, exchange repulsion, and orbital interaction, well beyond dimer systems. Beyond this, the correspondence of the individual orbitals to the initial monomer orbitals permits to assess how an interaction deforms an electron density. We show this for dipole moments, which may be decomposed into monomer contributions, polarization, and charge-transfer contribution. Applications to a water and an ammonia dimer and chains of water molecules show possible further developments toward multipolar expansions and other orbital-based schemes for parametrizing force fields.

Direct determination of localized Hartree–Fock orbitals as a step toward N scaling procedures

A method is proposed for the solution of the self-consistent field equations that can lead to localized occupied and virtual molecular orbitals, avoiding the need for solving for the canonical molecular orbitals. The method starts with strongly localized “guess molecular orbitals”, it is nonperturbative and proceeds through the diagonalization of single configuration interaction matrices which may be rendered size-consistent through appropriate coupled electron pair approximation or coupled-cluster-type dressings. We see a potential utility for the method in applications to large systems where localized orbitals will improve the scaling of the computational resources required with increasing system size.

Spin-unrestricted random-phase approximation with range separation: Benchmark on atomization energies and reaction barrier heights

We consider several spin-unrestricted random-phase approximation (RPA) variants for calculating correlation energies, with and without range separation, and test them on datasets of atomization energies and reaction barrier heights. We show that range separation greatly improves the accuracy of all RPA variants for these properties. Moreover, we show that a RPA variant with exchange, hereafter referred to as RPAx-SO2, first proposed by Szabo and Ostlund [J. Chem. Phys. 67, 4351 (1977)] in a spin-restricted closed-shell formalism, and extended here to a spin-unrestricted formalism, provides on average the most accurate range-separated RPA variant for atomization energies and reaction barrier heights. Since this range-separated RPAx-SO2 method had already been shown to be among the most accurate range-separated RPA variants for weak intermolecular interactions [J. Toulouse et al., J. Chem. Phys. 135, 084119 (2011)], this works confirms range-separated RPAx-SO2 as a promising method for general chemical applications.

Short range DFT combined with long-range local RPA within a range-separated hybrid DFT framework

Selecting excitations in localized orbitals to calculate long-range correlation contributions to range-separated density-functional theory can reduce the overall computational effort significantly. Beyond simple selection schemes of excited determinants, the dispersion-only approximation, which avoids counterpoise-corrected monomer caculations, is shown to be particularly interesting in this context, which we apply to the random-phase approximation. The approach has been tested on dimers of formamide, water, methane and benzene.

Improved version of a local contracted configuration interaction of singles and doubles with partial inclusion of triples and quadruples

A local contracted single and double configuration interaction (LC-CISD) method, which introduces contracted singly and doubly excited vectors within the framework of bond functions, has been recently proposed [P. Reinhardt et al., J. Chem. Phys. 129, 164106 (2008)]. The present work improves this method by introducing a coupled-electron pair approximation (CEPA-3) dressing and by incorporating the leading part of linked effects of triples (T) and quadruples (Q) through a series of local four-electron full CI calculations. Two different ways have been adopted to incorporate this linked TQ effect. One consists of dressing the first column/line of the whole LC-CISD matrix. The other one introduces an additional contracted wave function responsible for the linked effect for each bond pair. The present LC-CEPA-3+TQ treatments have been applied to the evaluation of equilibrium bond lengths and harmonic frequencies of diatomic molecules (HF, BF, CuH, N(2), F(2), and Cl(2)) and single bond breaking in HF, CH(4), ClCH(3), ClSiH(3), n-butane, and F(2) molecules, symmetrical stretching of the two OH bonds in a water molecule, and symmetrical expansion of a triangular Be(3) cluster. The results show that the performance of the LC-CEPA-3+TQs compares favorably with coupled-cluster singles and doubles (CCSD) and CCSD(T) methods, presenting similar behaviors around equilibrium and better ones for stretched geometries. The LC-CEPA-3 method is strictly separable, and the size consistency error of our treatment of triples and quadruples is extremely small. The strict separability can be further achieved by dressing the doubly excited bond functions with the linked TQ effect. The efficiency of truncations on the bielectronic integrals has also been tested.

Evidence of an isomeric pair in furan...HCl: Fourier transform infrared spectroscopy and ab initio calculations.

For the first time the coexistence of a sigma- and a pi-complex in the C(4)H(4)O:HCl system has been observed, in the same supersonic expansion of a molecular jet seeded with argon (or helium) or in a flow-cooled cell at 240 K. This is an exception to the third of the Legon-Miller rules which claims the sigma-structure to be the only one to exist. On the grounds of energetic considerations and band contour simulations, two observed bands at 2787.7 and 2795.5 cm(-1) of the nu(s) HCl stretching frequency are assigned to the two complexes, recorded as Fourier transform infrared spectra with a resolution between 0.2 and 0.5 cm(-1). Complementary calculations show that the use of the standard second-order Moller-Plesset perturbation theory may be erroneous for such a complex, due of the overestimation of the dispersion contribution with respect to the electrostatic term. It is finally established that only a balanced version of the second-order Moller-Plesset perturbation method, spin-component scaled-MP2, or a higher level of theory like a coupled-cluster approach, can provide a reliable energetic analysis for this complex.

Quantum Monte Carlo Calculations of Electronic Excitation Energies: The Case of the Singlet n→π∗ (CO) Transition in Acrolein

We report state-of-the-art quantum Monte Carlo calculations of the singlet n→π∗ (CO) vertical excitation energy in the acrolein molecule, extending the recent study of Bouabca et al. (J Chem Phys 130:114107, 2009). We investigate the effect of using a Slater basis set instead of a Gaussian basis set, and of using state-average versus state-specific complete-active-space (CAS) wave functions, with or without reoptimization of the coefficients of the configuration state functions (CSFs) and of the orbitals in variational Monte Carlo (VMC). It is found that, with the Slater basis set used here, both state-average and state-specific CAS(6,5) wave functions give an accurate excitation energy in diffusion Monte Carlo (DMC), with or without reoptimization of the CSF and orbital coefficients in the presence of the Jastrow factor. In contrast, the CAS(2,2) wave functions require reoptimization of the CSF and orbital coefficients to give a good DMC excitation energy. Our best estimates of the vertical excitation energy are between 3.86 and 3.89 eV.

Hydrogen isotope fractionation in methane plasma.

Significance Large variations in light element isotope ratios (H, N, C) are routinely observed in meteorite organic matter. The origin of these so-called anomalies is not accounted for by the classical theory of isotope fractionation. In the case of H, micrometer-size areas within the insoluble organic matter (IOM) isolated from meteorites by acid treatment, exhibit extreme deuterium enrichment. They are generally interpreted as components exogenous to the solar system and attributed to surviving interstellar grains. In the present paper, these H isotope anomalies were reproduced in the IOM synthesized through laboratory reactions involving CHx• radicals formed in CH4 plasma. Therefore, these anomalies may result from the chemistry occurring in the disk surrounding the Sun during its formation. The hydrogen isotope ratio (D/H) is commonly used to reconstruct the chemical processes at the origin of water and organic compounds in the early solar system. On the one hand, the large enrichments in deuterium of the insoluble organic matter (IOM) isolated from the carbonaceous meteorites are interpreted as a heritage of the interstellar medium or resulting from ion−molecule reactions taking place in the diffuse part of the protosolar nebula. On the other hand, the molecular structure of this IOM suggests that organic radicals have played a central role in a gas-phase organosynthesis. So as to reproduce this type of chemistry between organic radicals, experiments based on a microwave plasma of CH4 have been performed. They yielded a black organic residue in which ion microprobe analyses revealed hydrogen isotopic anomalies at a submicrometric spatial resolution. They likely reflect differences in the D/H ratios between the various CHx radicals whose polymerization is at the origin of the IOM. These isotopic heterogeneities, usually referred to as hot and cold spots, are commensurable with those observed in meteorite IOM. As a consequence, the appearance of organic radicals in the ionized regions of the disk surrounding the Sun during its formation may have triggered the formation of organic compounds.

Calculation of Zn, Cd, Hg adsorption on graphene with incremental CCSD(T) and range-separated hybrid DFT*

ABSTRACT At the hand of the adsorption of the metal atoms Zn, Cd and Hg on a graphene sheet, we propose a combination of range-separated hybrid density-functional theory in combination with the incremental scheme in localised orbitals and extrapolation procedures for the description of this type of extended systems. Using only dispersion terms for the long-range part, we were able to obtain results comparable to incremental coupled-cluster calculations with singles, doubles and perturbative triples (CCSD(T)). Repulsive three-centre increments reduce the overall correlation contribution to the binding energy by 20 %.