Electronic polarization effect on low-frequency infrared and Raman spectra of aprotic solvent: Molecular dynamics simulation study with charge response kernel by second order Møller-Plesset perturbation method Development of transferable interaction models for water. IV. A flexible, all-atom polarizable potential (TTM2-F) based on geometry dependent charges derived from an ab initio monomer dipole moment surfaceThe Ewald summation technique and the reaction field method have been generalized to potentials with atomic charges, dipole moments, and anisotropic polarizabilities. These are two common methods to treat long-range interactions in molecular simulations. Expressions for the potential energy, the electrostatic potential, the electrostatic field, the electrostatic field gradient, the force, and the virial are given, allowing for the calculation of long-range contributions to these properties within the Ewald summation or reaction field methods. We have compared numerical results using the Ewald summation under vacuum conditions with those from direct summations for a number of simple systems and found a complete agreement within the numerical precision with the exception of trivial shifts of the potential. The expressions given will facilitate the use of polarizable models in molecular simulations and hence improving our understanding of condensed matter.

The electronic spectrum of water is investigated using coupled cluster electronic structure methods. Vertical excitation energies are calculated for both gas phase H2O, various water clusters, as well as from models designed to simulate condensed phase effects. Four different approaches for describing condensed phase effects on the electronic transitions are investigated: continuum (a single water molecule embedded in a dielectric medium), discrete (water clusters), semidiscrete (a water pentamer cluster embedded in a dielectric medium), and intermolecular perturbation methods. The results are compared with experimental results. The solvent shift on the lowest state appears to be reasonably described by discrete and semidiscrete methods. It is very difficult to model the condensed-phase effects for the higher states of the pure liquid.

Nitromalonamide has a remarkably short intramolecular hydrogen bond with an O−O distance of 2.391(3) Å, which according to some expectations would indicate a symmetric hydrogen bond. However, the enol hydrogen is confirmed by neutron diffraction to have an asymmetric position between the two oxygen atoms in the otherwise quite symmetrical molecule. The O−H distances are 1.14(1) Å and 1.31(1) Å. Analysis of the atomic displacement parameters of the enol hydrogen and comparison with those of similar systems indicate that the hydrogen atom resides in a single well potential. The experimental structure is compared to geometry optimized structures obtained from high level ab initio computations and, except for the position of the enol hydrogen, generally good agreement is obtained when correlation is taken into account. The significant differences between experimental and ab initio O−H bond lengths are ascribed to dynamical and intermolecular effects. Extremely small proton transfer barriers of 0.6 kJ/mol at the MP2/cc-pVTZ and 1.2 kJ/mol at the B3LYP/cc-pVTZ levels of theory were calculated. The enol hydrogen is found to vibrate freely between the two oxygen atoms, without the molecule passing through a well refined transition state structure. A simple model of the crystal environment explains the asymmetry of the hydrogen bond as resulting from intermolecular hydrogen bonding.

The gas-to-liquid chemical shifts of water have been calculated by combining molecular dynamics simulations and quantum chemically derived shielding polarizabilities. The use of a force field based on intermolecular perturbation theory ensures that the electric fields are adequately modeled. The experimental proton shift and its temperature dependence are reproduced, but the oxygen shift lacks higher order terms such as the linear field-gradient contribution. Shifts arising from the difference in the gas phase and liquid geometries of the water molecule have been estimated and discussed.