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Three lowest diabatic potential energy surfaces of Br(2P)...H2(1SIGMA+) pre-reactive complex are studied using ab initio calculations. Three adiabatic potential energy surfaces obtained from the diabatic basis are also studied.

Interaction potentials of the iodine atom, atomic cation, and anion with light rare-gas atoms from He to Ar are calculated within the unified ab initio approach using the unrestricted coupled-cluster with singles and doubles and perturbative treatment of triples correlation treatment, relativistic small-core pseudopotential, and an extended basis set. Ab initio points are fit to a flexible analytical function. The calculated potentials are compared with available literature data, assessed in the I--and I+-ion mobility calculations and the Ar–I--anion zero electron kinetic-energy spectra simulations, and analyzed using the correlation rules. The results indicate a high precision of the reported potentials. [ABSTRACT FROM AUTHOR]

Three-body nonadditive effects in the Ar[sub 2]O[sup -] trimer have been calculated ab initio by means of the fourth-order Møller-Plesset perturbation theory, the coupled cluster theory with the single, double, and noniterative triple excitations, and the intermolecular Møller-Plesset perturbation approach. The genuine three-body effects have been separated from the two-body orientational nonadditivity. The latter arises in this system due to the rotation of the singly occupied p-symmetry orbital from the Σ and II symmetries in the dimer to the A[sub 1], B[sub 1], and B[sub 2] symmetries of the trimer. It is responsible for the collinear form of the trimer. In general, the total three-body effect has been found to be determined by the induction nonadditivity, but the exchange effects become important in the short range. Several models of the total three-body effect have been proposed and fitted to ab initio calculations. [ABSTRACT FROM AUTHOR]

The structure and energetics of the Ar[sub n]O[sup -] clusters for n=1, …,13 have been modeled in the framework of Diffusion Monte Carlo (DMC), using two- and three-body ab initio determined potentials derived previously by Buchachenko et al. [J. Chem. Phys. 112, 5852 (2000)], and Jakowski et al. [preceding paper, J. Chem. Phys. 118, 2731 (2003)], respectively. The anion cluster structures are largely determined by the two-body potential since the dominant contribution to the stabilization energy is due to pair interactions. However, the three-body effects are important since their role grows with n, from a few percent for n = 2 to ca. 30% for n = 12. The three-body effects are well approximated by the induction component only. The exchange and dispersion three-body and the induction many-body effects were found to be much less important. The effect of the spin-orbit coupling on the stabilization energies is small and almost independent of the size of a cluster. Specifically, it amounts to about 5% for ArO[sup -], and to 0.1% for Ar[sub 12]O[sup -]. The ab initio cluster stabilization energies are compared with those derived from the experimental measurements of electron detachment energies. The agreement is qualitatively good, and the origins of quantitative discrepancies are discussed. [ABSTRACT FROM AUTHOR]

The ab initio potential energy surface of the ArCO2 cluster is calculated using the supermolecular Mo\ller–Plesset perturbation theory (S-MPPT) and dissected into its fundamental components; electrostatic, exchange, induction, and dispersion energies. The surface contains a single minimum for the perpendicular approach of Ar toward the C atom which has a well depth of ∼210 cm-1 at R=6.5 a0. This value is obtained using an extended basis set supplied with the bond functions and the fourth order supermolecular Mo\ller–Plesset calculations, and is expected to be accurate to within ±5%. The areas of the surface corresponding to the collinear approach of Ar to CO2 contain an extended plateau. The saddle point in this region for R=9.0 a0 is stabilized by 117 cm-1. The analytical pair potential for Ar–CO2 obtained by fitting to the individual interaction components is provided. The three-body effects in the related cluster, Ar2CO2, are examined for two configurations of the Ar2CO2 cluster. The overall nonadditivity is dominated by the three-body dispersion effect; however, the exchange nonadditivity is the most anisotropic. The sources of this anisotropy are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The nonadditive effects are studied in Ar2HX (X=F, Cl) clusters using the supermolecular Mo\ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. The range of geometrical variations include the rotation of HX within the plane of the cluster and perpendicular to it. The three-body effect is rigorously dissected into the three fundamental nonadditive components: exchange, induction, and dispersion. The exchange nonadditivity is further divided into two terms: ES3, the electrostatic interaction between the HX and Ar2 subunits due to the appearance of exchange-induced quadrupole moment on Ar2 [Cooper and Hutson, J. Chem. Phys. 98, 5337 (1993)], and its exchange counterpart X3. ES3 is physically interpreted as originating from the interatomic electron exchanges within the Ar2 part. The X3 term originates from triple exchanges among all three monomers. The induction nonadditivity is dominated by the third-order effect describing the interaction of moments induced on the Ar atoms by the field of HX. This effect is faithfully represented by the multipole approximation. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 followed by dispersion and exchange effects. The dispersion effect is the largest in magnitude. The long-range part of the polarization nonadditivity is analyzed via the multipole approximation. The calculations indicate that the multipole expansion carried out to the inverse 12 power of R correctly describes the anisotropy of this effect. In Ar2HF the effect of intrasystem correlation on dispersion nonadditivity is also investigated via the supermolecular coupled cluster calculations. These calculations provide an estimate of 13.8 cm-1 for the total three-body effect in the equilibrium Ar2HF trimer. This value may still be underestimated due to the basis-set unsaturation of dispersion nonadditivity. [ABSTRACT FROM AUTHOR]

The potential energy surface of the H2CO–Ar complex is calculated at the second-order Mo\ller–Plesset perturbation theory and analyzed using the perturbation theory of intermolecular forces. The equilibrium geometry (De=171 cm-1) involves a T-shape structure with the Ar atom nearly perpendicular to the C–O bond of H2CO and in the molecular plane of H2CO. The equilibrium configuration results from a minimum in the exchange repulsion. It is conceivable that H2CO undegoes a hindered internal rotation in the complex. A barrier to such a motion is estimated at 32 cm-1. The potential energy surface is very complex. The anisotropy of the surface involving the in-plane motion of Ar is very high and resembles that of the Ar–H2O complex. The anisotropy involving motion of Ar perpendicular to the molecular plane of H2CO is much weaker, and it is strikingly similar to that of the Ar–CO complex. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 which sets the general trend for the anisotropy of the surface. A more detailed examination, however, indicates that the anisotropy of the surface results from a complex interplay of various factors, including electrostatics, exchange repulsion, and to a lesser degree, the deformation effects. Analysis of various exchangeless perturbation approximations to the deformation effect indicates that the neglect of exchange component of deformation may lead to an incorrect description of the van der Waals region. The analytical potential for the CH4-H2O interaction is provided. [ABSTRACT FROM AUTHOR]

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 and explain why NH3 is not capable of forming the H-bond structures to rare-gas atoms. [ABSTRACT FROM AUTHOR]

The perturbation theory of intermolecular forces in conjunction with the supermolecular Mo\ller–Plesset treatment is applied in the analysis of the potential energy surface of Ar–CH4. The anisotropy of the Ar–CH4 potential energy surface is chiefly due to exchange repulsion. In the equilibrium structure, Ar approaches the face of CH4 tetrahedron thus avoiding contacts with C–H bonds. The equilibrium Ar–C separation was found equal to 7.5 a0 and the De energy to 113 cm-1. We estimate that our De may be too small by up to 25% with respect to the accurate value. The properties of Ar–CH4 are also compared with other Ar-molecule systems, such as Ar–NH3, Ar–H2O, and Ar–HCl. We find that the equilibrium structures of weak proton donors bound to Ar (CH4, NH3) are determined by the exchange repulsion, while those of efficient proton donors, such as HCl (and to a lesser extent H2O), result from the strong polarization of Ar in the field of a molecule. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]