Richard J. Wheatley

An overlap model for estimating the anisotropy of repulsion

There is an urgent need for accurate anisotropic site-site intermolecular pair potentials for use in realistic simulations of the condensed phases and spectra of van der Waals molecules. However, all attempts to find an analytical form for the repulsion energy have relied on empirical models with many fitted parameters, whose determination requires large quantities of accurate experimental or ab initio data. In this paper, we develop and assess a procedure for estimating the repulsion anisotropy from the monomer wavefunctions which is straightforward, computationally inexpensive and capable of high accuracy. The method is based on the observation that the intermolecular repulsion energy is closely related to the overlap between the unperturbed charge densities of the interacting molecules. The overlap can be treated analytically, leading to an anisotropic site-site functional form. Model pair repulsion potentials with two sites are obtained for (F2)2 and (Cl2)2 by ignoring the bonding charge density. For ...

Intermolecular potential and second virial coefficient of the water-hydrogen complex

We construct a rigid-body (five-dimensional) potential-energy surface for the water-hydrogen complex using scaled perturbation theory (SPT). An analytic fit of this surface is obtained, and, using this, two minima are found. The global minimum has C2v symmetry, with the hydrogen molecule acting as a proton donor to the oxygen atom on water. A local minimum with Cs symmetry has the hydrogen molecule acting as a proton acceptor to one of the hydrogen atoms on water, where the OH bond and H2 are in a T-shaped configuration. The SPT global minimum is bound by 1097 microEh (Eh approximately 4.359744 x 10(-18) J). Our best estimate of the binding energy, from a complete basis set extrapolation of coupled-cluster calculations, is 1076.1 microEh. The fitted surface is used to calculate the second cross virial coefficient over a wide temperature range (100-3000 K). Three complementary methods are used to quantify quantum statistical mechanical effects that become significant at low temperatures. We compare our results with experimental data, which are available over a smaller temperature range (230-700 K). Generally good agreement is found, but the experimental data are subject to larger uncertainties.

Structure and vibrational spectra of methanol clusters from a new potential model

The structures and vibrational spectra of small methanol clusters from dimer to decamer have been calculated using a newly developed intermolecular potential which is essentially based on monomer wave functions. Special care has been taken for the description of the electrostatic interaction using a distributed multipole representation and including a penetration term. In addition, the potential model consists of repulsion, dispersion, and induction terms. Based on this potential model cluster structures have been calculated. The lowest energy dimer configuration is linear, while from trimer to decamer for the most stable structures ring configurations were found. Tetramer, hexamer, and octamer have S4-, S6-, and S8-symmetry, respectively. Vibrational spectra of the CO stretch and the OH stretch mode have been determined in the harmonic and in the anharmonic approximation using perturbation theory and variational calculations. Up to the tetramer the experimental spectra of the CO stretch mode are well rep...

The intermolecular potential energy surface for CO2–Ar: Fitting to high‐resolution spectroscopy of Van der Waals complexes and second virial coefficients

Two potential energy surfaces for CO2–Ar are obtained by least‐squares fitting to the high‐resolution spectra of Van der Waals complexes and the second virial coefficients of Ar+CO2 gas mixtures. The potentials incorporate a repulsive wall based on monomer ab initio calculations and the assumption that the repulsion potential is proportional to the overlap of the monomer charge densities. The dispersion energy is represented in a two‐site model, with dispersion centers located along the C–O bonds of CO2. The resulting potentials give a good representation of all the experimental data with only three or four adjustable parameters. They are quite different from previous empirical CO2–Ar potentials, which all have either a poor representation of the attractive well or a poor representation of the repulsive wall.

Gaussian multipoles in practice: electrostatic energies for intermolecular potentials

A method is presented for calculating the total electrostatic interaction energies between molecules from ab initio monomer wave functions. This approach differs from existing methods, such as Stones distributed multipole analysis (DMA), in including the short‐range penetration energy as well as the long‐range multipolar energy. The monomer charge densities are expressed as distributed series of atom‐centered functions which we call Gaussian multipoles; these are analogous to the distributed point multipoles used in DMA. Our procedure has been encoded in the GMUL program. Calculations have been performed on the formamide/formaldehyde complex, a model system for NH …︁ O hydrogen bonding in biological molecules, and also on guanidinium/benzene, modeling amino/aromatic interactions in proteins. We find that the penetration energy can be significant, especially in its contribution to the variation of the electrostatic energy with interaction geometry. A hybrid method, which uses Gaussian multipoles for short‐range atom pair interactions and point multipoles for long‐range ones, allows the electrostatic energies, including penetration, to be calculated at a much reduced cost. We also note that the penetration energy may provide the best route to an atom–atom anisotropic model for the exchange‐repulsion energy in intermolecular potentials.

Atomic charge densities generated using an iterative stockholder procedure

A simple computational technique is introduced for generating atomic electron densities using an iterated stockholder procedure. It is proven that the procedure is always convergent and leads to unique atomic densities. The resulting atomic densities are shown to have chemically intuitive and reasonable charges, and the method is used to analyze the hydrogen bonding in the minimum energy configuration of the water dimer and charge transfer in the borazane molecule.

A systematic intermolecular potential method applied to chlorine

A systematic method is described for finding effective model intermolecular pair potentials to use in realistic simulations of condensed phases. The pair potential is split up into electrostatic, dispersion and repulsion components, all of which may be parametrized by using ab initio monomer wavefunctions. The repulsion energy is estimated by a novel method that assumes proportionality to the overlap between the unperturbed charge densities. The resulting model includes atom-atom anisotropy in all three components of the potential in a realistic, theoretically justified manner. One parameter has to be obtained from elsewhere, in this case by fitting to solid state data. More parameters can be adjusted in order to absorb errors such as the neglect of many-body effects. Application of this method to chlorine gives a model with four fitted parameters that predicts many properties of the solid and liquid at least as accurately as the best modern potential, which has eight fitted parameters.

Intermolecular potentials and second virial coefficients of the water–neon and water–argon complexes

We construct potential-energy surfaces for the water–neon and water–argon complexes from scaled perturbation theory, and calibrate them using accurate supermolecule data. Our best estimates of the binding energies for these two systems are 66.9 and 142.7 cm−1, respectively, where the latter value is in good agreement with the spectroscopically determined AW2 potential. We calculate second virial coefficients, B12(T), and the related property φ12=B12−T(dB12/dT), and compare our results with experimental data for water–argon. The perturbation theory and AW2 B12(T) results are consistent, and demonstrate that current theoretical approaches yield more precise second virial coefficient data than any in the literature. Our φ12 calculations are in good agreement with experimental results derived from enthalpy-of-mixing data, though our estimated uncertainties are significantly smaller.

Dispersion energy damping functions, and their relative scale with interatomic separation, for (H, He, Li)-(H, He, Li) interactions

Accurate calculations of the non-expanded dispersion energies and damping functions, associated with the R -n multipolar dispersion energies for n ˇ- 10, are carried out for the six pair interactions arising from ground state H, He and Li. Highly correlated monomer wavefunctions, for both the relevant ground and excited states, are employed in the computations and the results are used to discuss the relative scale of the damping functions with interatomic separation. The results for the damping functions are also used to help assess the validity of various choices, in the literature, for empirical scaling parameters that connect the damping functions for a given interaction to those for H(1s)-H(1s) as a standard interaction and a new, effective, scaling procedure for heteronuclear dimers is proposed. The discussion also contains a rationalization of the trends in the scaling parameters, as a function of R, which is based on the extent to which the lowest S → L ≠ 0 transitions contribute to the dispersion ...

Intermolecular potential for the interaction of helium with ammonia

We develop an intermolecular potential for the interaction between helium and ammonia including flexibility in the ammonia inversion tunneling coordinate. The potential energy surface is generated by fitting to scaled perturbation theory calculations and is shown to be comparable with high-quality ab initio supermolecule calculations. We have characterized the potential energy surface for a number of ammonia geometries from planar to a highly distorted geometry. For all but the most distorted ammonia geometry, the global minimum has the helium atom in an equatorial location, equidistant from the two closest hydrogen atoms. As the ammonia molecule moves away from the planar configuration, the equatorial minima become less strongly bound while the binding energy increases in the axial regions of the potential energy surface. At the most distorted ammonia geometry, the equatorial minimum is a local minimum, and the global minimum has the helium atom on the symmetry axis of the molecule at the hydrogen end.