M. M. Szczesniak

Nonadditive effects in HF and HCl trimers

Nonadditive effects are calculated for (HF)3 and (HCl)3 complexes and analyzed via the combination of perturbation theory of intermolecular forces with Mo/ller–Plesset perturbation theory (MPPT). In both systems the nonadditivity is dominated by the self‐consistent field (SCF) deformation effect, i.e., mutual polarization of the monomer wavefunctions. Heitler–London exchange and correlation effects are of secondary importance. Three‐body terms exhibit much lesser basis set dependence than the two‐body effects and even quite moderate basis sets which are not accurate enough for treatment of two‐body forces can yield three‐body effects of quantitative quality. This is due in large measure to the additivity of strongly basis set dependent components such as uncorrelated and correlated electrostatics and dispersion. Various approximate models for the three‐body potentials and total interaction in the (HF)3 cluster are analyzed from the point of view of their ability to predict the orientation dependence of in...

Ab initio study of the intermolecular potential of Ar–H2O

The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential‐energy surface of Ar–H2O. The surface is very isotropic with the lowest barrier for rotation of ∼35 cm−1 above the absolute minimum. The lower bound for De is found to be 108 cm−1 and the complex reveals a very floppy structure, with Ar moving freely from the H‐bridged structure to the coplanar and almost perpendicular arrangement of the C2 –water axis and the Ar–O axis, ‘‘T‐shaped’’ structure. This motion is almost isoenergetic (energy change of less than 2 cm−1 ). The H‐bridged structure is favored by the attractive induction and dispersion anisotropies; the T‐shaped structure is favored by repulsive exchange anisotropy. The nonadditive effect in the Ar2–H2O cluster was also calculated. Implications of our results on the present models of hydrophobic interactions are also discussed.

Ab initio potential energy surface for the Ar(1S)+OH(X2Π) interaction and bound rovibrational states

Adiabatic potential energy surfaces for the 2A′ and 2A″ states of the Ar(1S)–OH(X2Π) complex were calculated using supermolecular unrestricted fourth-order Moller–Plesset perturbation theory and a large correlation consistent basis set supplemented with bond functions. The potential energy surface (PES) of the A′ state has two minima. The global minimum from the unrestricted coupled-cluster calculations with single, double, and noniterative triple excitations occurs for the collinear geometry Ar–H–O at R=7.08a0 with a well depth of De=141.2 cm−1. There is also a local minimum for the skewed T-shaped form, whereas the Ar–O–H arrangement corresponds to a saddle point. The PES of the A″ state also has two minima, which occur for the two collinear isomers. A variational calculation of the bound rovibrational states was performed. The calculated binding energy, D0=93.8 cm−1, and the energies of the bound vibrational states are in good agreement with experiment [see Berry et al., Chem. Phys. Lett. 178, 301 (199...

Paradigm pre-reactive van der Waals complexes: X–HX and X–H2 (X = F, Cl, Br)

This review describes recent progress in ab initio calculations and modeling of weak pre-reactive van der Waals complexes that appear in the entrance channels of benchmark atom–diatom reactions. Examples from recent work are used to demonstrate how relevant potential energy surfaces are obtained and modeled from first principles. The paradigm complexes include the X(2P)–HX and X(2P)–H2 (X = F, Cl, Br) systems, with O(3P)–HCl included for comparison. In these complexes an interaction with either the HX or the H2 molecule splits a degenerate P state of an open-shell atom into three potential energy surfaces, two of which are of the same symmetry. Application of state-of-the art highly correlated methods, CCSD(T) and MRCI, to the evaluation of adiabatic and diabatic states is discussed. Nonadiabatic coupling involving potential surfaces is compared for a number of complexes. Computational modeling of this term and its relationship to electrostatic interaction are also described. Spin–orbit coupling is shown to have dramatic effects on the structural and dynamic properties of these complexes.

Proton–donor properties of water and ammonia in van der Waals complexes with rare‐gas atoms. Kr–H2O and Kr–NH3

The perturbation theory of intermolecular forces in conjunction with the supermolecular Mo/ller–Plesset perturbation theory is applied to the analysis of the potential‐energy surfaces of Kr–H2O and Kr–NH3 complexes. The valleylike minimum region on the potential‐energy surface of Kr–H2O ranges from the coplanar geometry with the C2 axis of H2O nearly perpendicular to the O–Kr axis (T structure) to the H‐bond structure in which Kr faces the H atom of H2O. Compared to the previously studied Ar–H2O [J. Chem. Phys. 94, 2807 (1991)] the minimum has more of the H‐bond character. The minimum for Kr–NH3 corresponds to the T structure only, in accordance to the result for Ar–NH3 [J. Chem. Phys. 91, 7809 (1989)]. The minima in Kr–H2O and Kr–NH3 are roughly 27% and 19%, respectively, deeper than for the analogous Ar complexes. To examine the proton–donor abilities of O–H and N–H bonds the ratios of the deformation energy to dispersion energy are considered. They reflect fundamental differences between the two bonds ...

Ab initio study of the O2(X 3Σg−)+Ar(1S) van der Waals interaction

A potential energy surface for the Ar(1S)+O2(X 3Σg−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the T-shaped geometry, around 6.7 a0. Our UMP4 estimate of the well depth of the global minimum is De=117 cm−1 and the related ground state dissociation energy obtained by diffusion Monte Carlo calculations is 88 cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also reveals a local minimum for the collinear geometry at ca∼7.6 a0. The well depth for the secondary minimum at the UMP4 level is estimated at De=104 cm−1. The minima are separated by a barrier of 23 cm−1. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary, linear minimum is enhanced by a slight flattening of the electron density near the ends of the interoxyge...

Proton-donor properties of water and ammonia in van der Waals complexes. Be-H2O and Be-NH3

The potential energy surfaces (PES) of Be–H2O and Be–NH3 are studied with particular attention to characterization of proton‐donor properties of water and ammonia. Calculations were performed by means of both supermolecular and intermolecular Mo/ller Plesset perturbation theory. The Be–H2O PES reveals two van der Waals minima: the C2v minimum (De=176 cm−1, Re=6.5 bohr), and the H‐bonded minimum (De=161 cm−1, Re=7.5 bohr), separated by a barrier of 43 cm−1 at the T‐shaped configuration. The Be–NH3 PES reveals only one van der Waals minimum, at the C3v configuration (De=260 cm−1, Re=6.5 bohr) and a saddle point at the H‐bonded geometry. The locations of the minima as well as the anisotropy of the interaction are determined by the anisotropy of electric polarization contribution, embodied by the self‐consistent‐field (SCF)‐deformation and perturbation induction energies.

Employing Range Separation on the meta-GGA Rung: New Functional Suitable for Both Covalent and Noncovalent Interactions

We devise a scheme for converting an existing exchange functional into its range-separated hybrid variant. The underlying exchange hole of the Becke-Roussel type has the exact second-order expansion in the interelectron distance. The short-range part of the resulting range-separated exchange energy depends on the kinetic energy density and the Laplacian even if the base functional lacks the dependence on these variables. The most successful practical realization of the scheme, named LC-PBETPSS, combines the range-separated Perdew-Burke-Ernzerhof (PBE) exchange lifted to the hybrid meta-generalized gradient approximation rung and the Tao-Perdew-Staroverov-Scuseria (TPSS) correlation. The value of the range-separation parameter is estimated theoretically and confirmed by empirical optimization. The D3 dispersion correction is recommended for all energy computations employing the presented functional. Numerical tests show remarkably robust performance of the method for noncovalent interaction energies, barrier heights, main-group thermochemistry, and excitation energies.