Ab initio study of intermolecular potential of H2O trimer

Nonadditive contribution to the interaction energy in water trimer is analyzed in terms of Heitler–London exchange, SCF deformation, induction and dispersion nonadditivities. Nonadditivity originates mainly from the SCF deformation effect which is due to electric polarization. However, polarization does not serve as a universal mechanism for nonadditivity in water. In the double‐donor configuration, for example, the Heitler–London exchange contribution is the most important and polarization yields the wrong sign. Correlation effects do not contribute significantly to the nonadditivity. A detailed analysis of the pair potential is also provided. The present two‐body potential and its components are compared to the existing ab initio potentials (MCY) as well as to empirical ones (RWK2,TIP,SPC). The ways to improve these potentials are suggested

Ab initio study of the intermolecular potential of Ar–H2O

The combination of supermolecular Møller–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

Scattering length of the ground state Mg+Mg collision

We have constructed the X 1SIGMAg+ potential for the collision between two ground state Mg atoms and analyzed the effect of uncertainties in the shape of the potential on scattering properties at ultra-cold temperatures. This potential reproduces the experimental term values to 0.2 inverse cm and has a scattering length of +1.4(5) nm where the error is prodominantly due to the uncertainty in the dissociation energy and the C6 dispersion coefficient. A positive sign of the scattering length suggests that a Bose-Einstein condensate of ground state Mg atoms is stable.Comment: 15 pages, 3 figures, Submitted Phys. Rev.

Tractable non-local correlation density functionals for flat surfaces and slabs

A systematic approach for the construction of a density functional for van der Waals interactions that also accounts for saturation effects is described, i.e. one that is applicable at short distances. A very efficient method to calculate the resulting expressions in the case of flat surfaces, a method leading to an order reduction in computational complexity, is presented. Results for the interaction of two parallel jellium slabs are shown to agree with those of a recent RPA calculation (J.F. Dobson and J. Wang, Phys. Rev. Lett. 82, 2123 1999). The method is easy to use; its input consists of the electron density of the system, and we show that it can be successfully approximated by the electron densities of the interacting fragments. Results for the surface correlation energy of jellium compare very well with those of other studies. The correlation-interaction energy between two parallel jellia is calculated for all separations d, and substantial saturation effects are predicted.Comment: 10 pages, 6 figure

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 Møller–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

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 Møller 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

An analysis of the partial wave expansion of the dispersion energy for Ne2

Calculations of the dispersion energy for Ne2 by using the partial wave expansion through the h=h term were carried out in the region of the van der Waals minimum. It is shown that, at R = 6ag4, lack of higher than d orbitals gives a dispersion energy in error by ≈16%, and lack of higher than f orbitals gives an error of ≈6%

Ab initio study of He(1S)+Cl2(X 1Σg,3Πu) potential energy surfaces

The potential energy surface of the ground state He+Cl2(1Σg) is calculated by using the perturbation theory of intermolecular forces and supermolecular Møller–Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl2(3Πu) was evaluated using the supermolecular unrestricted Møller–Plesset perturbation theory approach. In the ground state two stable isomers are found which correspond to the linear He–Cl–Cl structure (a primary minimum, De=45.1 cm−1, Re=4.25 Å) and to the T‐shaped structure with He perpendicular to the molecular axis (a secondary minimum, De=40.8 cm−1, Re=3.5 Å). The small difference between these geometries is mainly due to the induction effect which is larger for the linear form. The results obtained for the T‐shaped minimum are in good agreement with the excitation spectroscopy experiments which observed only the T‐shaped form [Beneventi et al., J. Chem. Phys. 98, 178 (1993)]. In the lowest triplet states correlating with Cl2(3Πu), 3A′ and 3A″, the same two isomers correspond to minima. Now, however, the T‐shaped form is lower in energy. The 3A′ and 3A″ states correspond to (De,Re) of (19.9 cm−1, 3.75 Å) and (30.3 cm−1, 3.50 Å), respectively, whereas the linear form is characterized by (19.8 cm−1, 5.0 Å). The binding energy for the T form in the lower 3A″ state is in good agreement with the experimental value of Beneventi et al

Site-Site Function and Successive Reaction Counterpoise Calculation of Basis Set Superposition Error for Proton Transfer

Basis set superposition error (BSSE) is computed for the hydrogen-bonded system (FH…F)− by the site-site function (SSFC) and successive reaction counterpoise (SRCP) methods using a wide array of basis sets, at SCF and correlated levels up through MP4. For the complexation process the BSSE can be fairly large. However, as it is comparable in magnitude for the endpoint and midpoint of the proton transfer, the BSSE has only a very small effect upon the barrier to proton transfer for all basis sets. SSFC and SRCP results are generally similar to those obtained with the standard Boys-Bernardi scheme