M. M. Szczȩśniak

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.

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.

Ab initio study of intermolecular potential for ArHCl

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 ArHCl. Two minima have been found, a primary for collinear Ar–HCl and a secondary for collinear Ar–ClH. The depths of these minima are about 12% below the empirical estimates, due to basis set unsaturation of the dispersion contribution. The Ar–HCl structure is favored by the induction and dispersion terms whereas Ar–ClH by the exchange–repulsion term. The total ab initio potential, as well as its components, are compared with related terms of recent Hutson’s H6(3) potential [J. Chem. Phys. 89, 4550 (1988)] and the anisotropy of interaction is analyzed. It is concluded that the one‐center multipole expansions of induction and dispersion contributions do not reproduce the correct anisotropy of induction and dispersion terms. Ab initio estimates of three‐body effects in the Ar2HCl complex are also discussed.

The nonadditive interactions in the Ar2HF and Ar2HCl clusters: An ab initio study

The three‐body effects in the Ar2HX (X=F, Cl) are studied by means of the supermolecular Mo/ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. In both systems the nonadditive interactions are large and repulsive around the equilibrium geometry. The in‐plane bending potential of H–F in the Ar2HF cluster reveals a double minimum with the barrier of ca. 2–3 cm−1. The barrier is due to the three‐body interactions. In Ar2HCl the analogous potential has a single minimum, and the three‐body effects make it shallower. The three‐body interaction energy is dissected into its components such as exchange, polarization, and dispersion. The anisotropy of the total nonadditvity in Ar2HF is dominated by polarization and exchange effects, and, consequently, it can be well reproduced at the self‐consistent field level of theory. The overall nonadditivity in Ar2HCl is quite similar in magnitude, but it displays a different composition. The most anisotropic is polarization ...

Partitioning of interaction energy in van der Waals complexes involving excited state species: The He(1S)+Cl2(B 3Πu) interaction

The partitioning of interaction energy between a closed‐shell and an open‐shell system is proposed. This allows us to describe the unrestricted Mo/ller–Plesset interaction energy as a sum of fundamental contributions: electrostatic, exchange, induction and dispersion. The supermolecular energies derived within unrestricted Mo/ller–Plesset perturbation theory are analyzed in terms of perturbation theory of intermolecular forces. The latter has been generalized to allow for the description of monomer wave functions within the unrestricted Hartree–Fock approach. The method is applied to the potential energy surfaces for the first excited triplet states, 3A′ and 3A″, of the He+Cl2(3Πu) complex. The 3A′ and 3A″ potential energy surfaces have different shapes. The lower one, 3A′, has a single minimum for the T‐shaped structure. The higher one, 3A″, has the global minimum for the T‐shaped structure and the secondary minimum for a linear orientation. The calculated well depth for the 3A′ state is 31.1 cm−1 at the...

Ab initio potential energy and dipole moment surfaces, infrared spectra, and vibrational predissociation dynamics of the 35Cl−⋯H2/D2 complexes

Three-dimensional potential energy and dipole moment surfaces of the Cl−–H2 system are calculated ab initio by means of a coupled cluster method with single and double excitations and noniterative correction to triple excitations with augmented correlation consistent quadruple-zeta basis set supplemented with bond functions, and represented in analytical forms. Variational calculations of the energy levels up to the total angular momentum J=25 provide accurate estimations of the measured rotational spectroscopic constants of the ground van der Waals levels n=0 of the Cl−⋯H2/D2 complexes although they underestimate the red shifts of the mid-infrared spectra with v=0→v=1 vibrational excitation of the monomer. They also attest to the accuracy of effective radial interaction potentials extracted previously from experimental data using the rotational RKR procedure. Vibrational predissociation of the Cl−⋯H2/D2(v=1) complexes is shown to follow near-resonant vibrational-to-rotational energy transfer mechanism so...

Ab initio calculations of the interaction of He with the B 3Π0u+ state of Cl2 as a function of the Cl2 internuclear separation

Ab initio calculations using unrestricted Mo/ller–Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B 3Π0u+ state of Cl2. Also, more reliable unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl–Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B←X, 8←0 spectrum is in good agree...

Counterpoise-corrected geometries and harmonic frequencies of N-body clusters: Application to (HF)n (n=3,4)

The differences between three previously defined counterpoise (CP) schemes for removing the BSSE in molecular complexes formed by more than two subunits have been assessed by CP-corrected geometry optimizations and frequency calculations for the hydrogen fluoride trimer and tetramer. The types of the functional counterpoise (FC) procedures included the site–site (SSFC), pairwise additive, and hierarchical Valiron–Mayer (VMFC) schemes. The latter approach takes into account the basis set extension of the dimers in the trimer, dimers and trimers in the tetramer, etc. The number of different calculations required to apply this counterpoise scheme increases very rapidly with the cluster size. The symmetry of the chosen systems makes the test of this approach computationally feasible. All the optimizations and frequency calculations have been carried out automatically using a new program that generates the necessary input files and repeatedly calls a slightly modified version of a Gaussian link. The results sh...

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.

Collision and transport properties of Rg+Cl(2P) and Rg+Cl−(1S) (Rg=Ar, Kr) from ab initio potentials

Highly accurate ab initio coupled cluster theory calculations, with single, double and noniterative triple excitations [CCSD(T)], and with the extended basis set augmented by the bond functions, were performed for the interactions of chlorine atom and chloride anion with Ar and Kr. Analytical fits to the ab initio points were shown to provide the consistent and reliable set of multiproperty potentials capable of reproducing all the available experimental data, namely: Total cross sections for Rg+Cl, reduced mobilities and diffusion coefficients for Rg+Cl−, as well as the data of spectroscopic zero electron kinetic energy experiments [see Buchachenko et al., J. Chem. Phys. 114, 9929 (2001), following paper]. The relative accuracy of available interaction potentials is discussed.