Total intermolecular interaction energies are determined with a first version of the Gaussian electrostatic model (GEM-0), a force field based on a density fitting approach using s-type Gaussian functions. The total interaction energy is computed in the spirit of the sum of interacting fragment ab initio (SIBFA) force field by separately evaluating each one of its components: electrostatic (Coulomb), exchange repulsion, polarization, and charge transfer intermolecular interaction energies, in order to reproduce reference constrained space orbital variation (CSOV) energy decomposition calculations at the B3LYP/aug-cc-pVTZ level. The use of an auxiliary basis set restricted to spherical Gaussian functions facilitates the rotation of the fitted densities of rigid fragments and enables a fast and accurate density fitting evaluation of Coulomb and exchangerepulsion energy, the latter using the overlap model introduced by Wheatley and Price [Mol. Phys. 69, 50718 (1990)]. The SIBFA energy scheme for polarization and charge transfer has been implemented using the electric fields and electrostatic potentials generated by the fitted densities. GEM-0 has been tested on ten stationary points of the water dimer potential energy surface and on three water clusters (n=16,20,64). The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below k B T at room temperature. Preliminary results for Coulomb and exchange-repulsion energies of metal cation complexes and coupled cluster singles doubles electron densities are discussed.

The hydration free energy, structure, and dynamics of the zinc divalent cation are studied using a polarizable force field in molecular dynamics simulations. Parameters for the Zn 2+ are derived from gas-phase ab initio calculation of Zn 2+ -water dimer. The Thole-based dipole polarization is adjusted based on the Constrained Space Orbital Variations (CSOV) calculation while the Symmetry Adapted Perturbation Theory (SAPT) approach is also discussed. The vdW parameters of Zn 2+ have been obtained by comparing the AMOEBA Zn 2+ -water dimerization energy with results from several theory levels and basis sets over a range of distances. Molecular dynamics simulations of Zn 2+ solvation in bulk water are subsequently performed with the polarizable force field. The calculated first-shell water coordination number, water residence time and free energy of hydration are consistent with experimental and previous theoretical values. The study is supplemented with extensive Reduced Variational Space (RVS) and Electron Localization Function (ELF) computations in order to unravel the nature of the bonding in Zn 2+ (H 2 O) n (n=1,6) complexes and to analyze the charge transfer contribution to the complexes. Results show that the importance of charge transfer decreases as the size of Zn-water cluster grows due to anticooperativity and to changes in the nature of the metal-ligand bonds. Induction could be dominated by polarization when the system approaches condensed-phase and the covelant effects are eliminated from the Zn(II)-water interaction. To construct an "effective" classical polarizable potential for Zn 2+ in bulk water, one should therefore avoid over-fitting to the ab initio charge transfer energy of Zn 2+ -water dimer. Indeed, in order to avoid overestimation of condensed-phase many-body effects, which is crucial to the transferability of polarizable molecular dynamics, charge transfer should not be included within the classical polarization contribution and should preferably be either incorporated in to the pairwise van der Waals contribution or treated explicitly.

Electron attachment and cesium collisional ionization studies of tetrafluorosuccinic and hexafluoroglutaric anhydrides: Molecular electron affinitiesThe negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO" COS, and CS, have been studied from threshold to -400 eV (lab). Ions with masses corresponding to the parent molecules CO" COS, and CS, are detected for all collision permutations except for Na colliding with CO,. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: COl-O.60±O.2 eV), COS( +0.46±0.2 eV), and CS,(l.O±O.2 eV). The CO 2 ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO, and in agreement with our earlier observations of CO 2 • and with recent theoretical calculations. The lifetime of CO 2 • (9±2X 10-5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to -20 eV c.m.). The fragment ions O-/CO" O-/COS, S-/COS, and S-/CS, were detected at a threshold energy which is consistent with the known energetics of the reactions. These results are compared with recent dissociative electron attachment studies.