Grzegorz Chałasiński

Proper correction for the basis set superposition error in SCF calculations of intermolecular interactions

The effects of basis set extension to the space of the partner monomer in supermolecular SCF and perturbation calculations have been analysed for the model He2 and HeLi+ systems. In contrast to the suggestions of many authors it is demonstrated that the correction for the basis set superposition error should be calculated with the full basis set of the dimer without removing the occupied orbitals of the partner monomer. In fact, the enlargement of the monomer basis set by the partner occupied orbitals effectively improves the first-order exchange interaction energy. For the first-order electrostatic interaction energy any enlargement of a poor or medium size basis set of the monomer by the partner orbitals worsens its value.

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.

Correlation correction to the Hartree-Fock electrostatic energy including orbital relaxation

Abstract By using the recently developed density approach to the analytic MBPT (2) gradients we derive the spin-adapted form of the correlation correction to the Hartree-Fock electrostatic energy using the relaxed MBPT (2) density matrix. This correction is shown to be essential for a perturbation theory analysis of the MBPT (2) supermolecular interaction energy at large distances. Applications are presented for various configurations of the H2 dimer and the results are compared to the full CI electrostatic energy data.

Perturbation analysis of the supermolecule interaction energy and the basis set superposition error

Abstract The supermolecule interaction energies at the self-consistent field and the second-order Moller—Plesset perturbation theory levels are analyzed using the polarization approximation perturbation theory of intermolecular interactions. The results for the HeH + complex show that the perturbation expansion converges and its sum is identical with the supermolecule interaction energy. The counterpoise-corrected supermolecule interaction energies for various complexes in the region of small intermolecular overlap are in excellent agreement with the perturbation results for both the basis sets of monomers and the basis set of the entire complex. Comparison of the results obtained with large and small basis sets for the HeH + and HeLi + complexes is used to emphasize the need to remove the basis set superposition error and to demonstrate the correctness of the counterpoise procedure.

Ab initio calculations of nonadditive effects

Abstract Supremolecular Moller-Plesset perturbation theory in conjunction with intermolecular Moller-Plesset perturbation theory permits the dissection of the nonadditive three-body interaction into physically meaningful contributions, such as exchange, induction, deformation, and dispersion. Recently, such a partitioning has been performed for a number of trimers, such as polar ((HF) 3 ,(HCl) 3 , (H 2 O) 3 ,(NH 3 ) 3 ), nonpolar ((CH 4 ) 3 ), and Ar-containing clusters (Ar 2 HCl, Ar 2 H 2 O, Ar 3 ). The present paper attempts to compare the nonadditive properties across this wide spectrum of interacting systems. The following nonadditive terms: Heitler-London-exchange, SCF-deformation, dispersion, and second-order exchange are analysed, and their role in the overall three-body potentials is discussed.

Perturbation calculations of the interaction energy between closed-shell Hartree-Fock atoms

Interaction energy between two neon atoms described by the Hartree-Fock wavefunctions has been calculated through the second order of the symmetry-adapted perturbation theory. Particular contributions, the first order energy, dispersion, induction, exchange-dispersion and exchange-induction energies, are analysed and the basis set effects are discussed. The calculated values of the interaction energies are close to the experimental energies.

Ab initio study of the van der Waals interaction of NH(X 3Σ−) with Ar(1S)

The potential energy surface for the Ar(1S)+NH(X 3Σ−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory approach and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the approximate T-shaped geometry with Ar skewed toward the H atom at about Θ=67° and R=6.75 a0. Our UMP4 estimate of the well depth of the global minimum is De=100.3 cm−1 and the related ground state dissociation energy obtained by rigid-body diffusion quantum Monte Carlo calculations (RBDQMC) is D0=71.5±0.1 cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also features a wide plateau in the proximity of Ar-N-H collinear geometry, at ca. 7.0 a0. RBDQMC calculations reveal nearly a free rotation of the NH subunit in the complex.

Ab initio based study of the ArO− photoelectron spectra: Selectivity of spin–orbit transitions

A combined ab initio atoms-in-molecule approach was implemented to model the photoelectron spectra of the ArO− anion. The lowest adiabatic states of Σ and Π symmetry of ArO and ArO− were investigated using the fourth-order Moller–Plessett perturbation theory including bond functions. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The complex of Ar with atomic oxygen is only weakly bound, primarily by dispersion interaction. The Π state possesses a deeper minimum (Re=3.4 A,De=380 μEh) than the Σ state (Re=3.8 A,De=220 μEh). In contrast, the anion complex is fairly strongly bound, primarily by ion-induced dipole induction forces, and the Σ state possesses a deeper minimum at shorter interatomic distances (Re=3.02 A,De=3600 μEh) than the Π state (Re=3.35 A,De=2400 μEh). The Σ–Π splittings in both systems are mainly due to differences in the exchange repulsion terms. Atoms-in-molecule models were used to account for the spin–orbit interaction, and to gene...

On the nature of the interaction energy in the Ar-Cl2 complex

Abstract An analysis of the potential energy surface of the Ar-Cl 2 complex was performed using the perturbation theory of intermolecular forces and supermolecular MPPT approach. Two types of minimum were found which correspond to the linear Ar⋯Cl-Cl structure (a primary minimum, D e = 222 cm −1 , R e = 4.5 A) and to the T-shaped structure with the argon atom perpendicular to the molecular axis (a secondary minimum, D e = 207 cm −1 , R e = 3.8 A). The small difference between the D e values does not exceed 16 cm −1 and is mainly due to the induction effects which are larger for the linear form. Although the linear form is lower in energy, the T-shaped isomer may be more stable because its zero-point energy is smaller than that of the linear form by about 18 cm −1 (F.-M. Tao and W. Klemperer, J. Chem. Phys., 97 (1992) 440). Indeed, only the T-shaped form is detected in the excitation and microwave spectroscopy experiments. It is also found that the electron density distribution in the Cl 2 case may be visualized as a dumbbell with flattened ends. By comparison, the ClF distribution can be viewed as an unsymmetrical dumbbell with a dent at the chlorine atom.