Sławomir M. Cybulski

Ground state potential energy curves for He2, Ne2, Ar2, He–Ne, He–Ar, and Ne–Ar: A coupled-cluster study

Potential energy curves for three homonuclear (He2, Ne2, Ar2) and three heteronuclear (He–Ne, He–Ar, Ne–Ar) rare gas dimers are presented. The curves were calculated using several correlation consistent basis sets and the supermolecule single and double excitation coupled-cluster theory with noniterative perturbational treatment of triple excitations, CCSD(T). The most accurate results were obtained with the aug-cc-pV5Z basis set supplemented with an additional (3s3p2d2f1g) set of bond functions. The results obtained with a smaller aug-cc-pVQZ+(3s3p2d2f1g) basis set are almost as accurate. Both basis sets give results in better agreement with potentials based on experiments than the recent results obtained with larger d-aug-cc-pV6Z and t-aug-cc-pV6Z basis sets but without bond functions. For each complex and each basis set a fitted potential energy curve is given. In addition, for each complex, with the exception of He2, the values of Re, De, B0, D0, and 〈R〉0 are given. For He2 no bound states were found ...

On decomposition of second‐order Mo/ller–Plesset supermolecular interaction energy and basis set effects

The basis set effects on the total self‐consistent field (SCF) and second‐order Mo/ller–Plesset (MP2) interaction energies in the HF dimer (in the equilibrium geometry) are investigated in relation to their components: electrostatic, exchange, induction, and dispersion, calculated within the framework of intermolecular Mo/ller–Plesset perturbation theory (IMPPT). The basis set dependence of the SCF interaction energy in the HF dimer is almost exactly determined by the electrostatic contribution. The exchange, induction, and the SCF‐deformation terms are found substantially less sensitive. The MP2 correlation contribution reflects primarily the basis set dependence of dispersion. However, an accurate image of the basis set dependence is reproduced only if the electrostatic‐correlation term is considered as well. Other correlation contributions: the deformation‐ correlation and exchange terms are found to be much less sensitive to basis set effects. All these conclusions are valid only under the condition t...

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...

The origin of deficiency of the supermolecule second-order Møller-Plesset approach for evaluating interaction energies

Calculations for the complex of thymine and adenine are used to show that the supermolecule second-order Moller-Plesset perturbation theory (MP2) approach for evaluating interaction energies fails in certain cases because of the behavior of one of its components: the uncoupled Hartree-Fock dispersion energy. A simple approach for correcting the MP2 supermolecule interaction energies is proposed. It focuses on correcting a relatively small difference between the MP2 and coupled cluster interaction energies, which is a very appealing feature of the new approach considering a benchmark role played by coupled cluster results.

Calculations of nonadditive effects by means of supermolecular Mo/ller–Plesset perturbation theory approach: Ar3 and Ar4

Nonadditive, multibody effects arising in the supermolecular Mo/ller–Plesset perturbation theory (MPPT) (IMPPT) calculations are classified and interpreted in terms of the exchange, induction, deformation, and dispersion contributions, as defined by the perturbation theory of intermolecular forces. As an example the many‐body effects in the equilateral Ar trimer and tetrahedral Ar tetramer, calculated through the MP4 level of theory with extended basis [7s4p2d], are reported and discussed. It is stressed that the ‘‘Heitler–London‐exchange plus dispersion’’ model for nonadditive effects is too attractive mainly because of the neglect of the second‐order exchange contribution.

Ab initio study of the potential energy surface of CH4‐H2O

The potential energy surface of CH4‐H2O is calculated through the fourth‐order Mo/ller–Plesset perturbation theory. In an attempt to obtain basis‐set saturated values of interaction energies the extended basis sets are augmented by bond functions which simulate the effects of high‐symmetry polarization functions. The absolute minimum occurs for the configuration involving the C–H‐O hydrogen‐bond in which O‐H points toward one of the faces of the CH4 tetrahedron. The equilibrium C–O separation is equal to 6.8 a0 which corresponds to the bond energy of 0.83 kcal/mol. Due to basis set unsaturation of the dispersion energy the bond energy may still be underestimated by about 0.05 kcal/mol. The secondary minimum involving the C‐H–O hydrogen‐bond is some 0.2 kcal/mol less stable, and the corresponding C–O distance is longer by 0.6 a0. The anisotropy of the potential energy surface is analyzed via the perturbation theory of intermolecular forces. The binding in CH4‐H2O is chiefly due to the dispersion energy whi...

Analysis of the potential energy surface of Ar–NH3

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–NH3. Anisotropy of the self‐consistent field (SCF) potential is determined by the first‐order exchange repulsion. Second‐order dispersion energy, the dominating attractive contribution, is anisotropic in the reciprocal sense to the first‐order exchange, i.e., minima in one nearly coincide with maxima in the other. The estimated second‐order correlation correction to the exchange effect is nearly as large as a half ΔESCF in the minimum and has a ‘‘smoothing’’ effect on the anisotropy of e(20)disp. The model which combines ΔESCF with dispersion energy (SCF+D) is not accurate enough to quantitatively describe both radial and angular dependence of interaction energy. Comparison is also made between Ar–NH3 and Ar–PH3, as well as with the Ar dimer.

Comparison of Morokuma and perturbation theory approaches to decomposition of interaction energy. (NH4)+…NH3

Abstract The interaction energy between NH 4 + and NH 3 is decomposed for a range of intermolecular separations within the context of a variety of basis sets including 6-31G ** , 6-31+G ** , and Sadlej polarization sets of type [5s3p2d/3s2p], including a set of f-functions as well. The electrostatic component dominates the interaction in all cases. At separations near the minimum or closer, the Morokuma second-order components (POL and CT) blow up. In contrast, Δ E SCF def , representing mutual polarization and orbital relaxation, is in good agreement with the induction energy computed via perturbation theory. Correction of the superposition error is noted to be important for this agreement and, indeed, for all components. In fact, following this correction, even the smaller sets provide accurate measures of the exchange term. The second-order correlation correction to the electrostatic interaction ϵ es 12 is computed and found to be fairly small. The dispersion energy embodied in ϵ disp 20 is considerably larger than the latter term and grows in proportion to the flexibility of the basis set.

Intermolecular Potential of the Methane Dimer and Trimer

The Heitler–London (HL) exchange energy is responsible for the anisotropy of the pair potential in methane. The equilibrium dimer structure is that which minimizes steric repulsion between hydrogens belonging to opposite subsystems. Dispersion energy, which represents a dominating attractive contribution, displays an orientation dependence which is the mirror image of that for HL exchange. The three‐body correction to the pair potential is a superposition of HL and second‐order exchange nonadditivities combined with the Axilrod–Teller dispersion nonadditivity. A great deal of cancellation between these terms results in near additivity of methane interactions in the long and intermediate regions.

An ab initio study of the potential energy surface and spectrum of Ar–CO

The two-dimensional potential energy surfaces for the Ar–CO complex have been developed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)]. The most accurate results have been obtained with the augmented correlation-consistent polarized triple zeta basis set (aug-cc-pVTZ) with an additional (3s3p2d2f1g) set of bond functions. The minimum of −104.68 cm−1 has been found at (R,Θ)=(3.714 A, 92.88°), where R and Θ denote the Jacobi coordinates with Θ=0° corresponding to the linear Ar–OC geometry and Θ=180° to the linear Ar–CO geometry. Dynamical calculations have been performed to determine the frequencies of various rotational and rovibrational transitions. The overall agreement with experiment is good. For example, the calculated frequencies of the intermolecular bending and stretching vibrations, 12.015 and 18.520 cm−1, respectively, agree very well with the experimental values (12.014 and 18.110 cm−1).