Simulations are presented of H2O ice, liquid, and clusters (H2O) n n ≤ 7. The first part is devoted to orientational energetics of ice. Ordinary hexagonal ice is orientationally disordered; a transition to an ordered form (ice XI) can be induced at 72 K. The recently demonstrated ferroelectric structure of ice XI (Jackson, S. M.; Wield, V. M.; Whitworth, R. W.; Oguro, M.; Wilson, C. J. Phys. Chem. B 1997, 101, 6142) seems to contradict our understanding of H2O···H2O interactions. A polarizable water potential is proposed that accounts qualitatively for the existence of ferroelectric ice XI; its crucial ingredient is the location of the polarizability center. This potential is then tested in classical trajectory simulations of water structure, energetics, self-diffusion, and dielectric constant. Cluster energetics and rotational constants are calculated using the rigid body diffusion Monte Carlo technique.

We present an analysis of the complex of water with CO which includes (a) a new potential energy surface obtained by fitting ab initio points, followed by adjustment against experimental rotational, spin–spin and quadrupole coupling constants; (b) diffusion Monte Carlo (DMC) studies of the ground vibrational state, and of three excited vibrational states, at J=0. A new approach is suggested for DMC calculation of intermolecular vibrational frequencies.

We present an analysis of the complex of water with N2 which includes (a) ab initio calculations of the potential energy surface; (b) ab initio analysis of the different contributions to the interaction energy (exchange, electrostatic, dispersion, induction); (c) a diffusion Monte Carlo study of the vibrational ground state and a calculation of vibrationally averaged spectroscopic constants for the different isotopic species; (d) construction of an N2⋅⋅H2O potential function by fitting to ab initio points followed by adjustment against the experimental rotational and quadrupole coupling constants of Leung et al. [J. Chem. Phys. 90, 700 (1989)].

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Intramolecular fundamentals of CO⋅⋅H2O are studied using a combination of diffusion Monte Carlo (DMC), and an adiabatic approximation based on frequency separation between inter- and intramolecular vibrations. Variational treatment in a Morse basis is employed for the stretch excitations. DMC is used to calculate the distribution of intermolecular configurations, and to evaluate the (substantial) influence of intermolecular zero-point motion on intramolecular frequency shifts. The dependence of the potential on intermolecular coordinates was derived with the help of ab initio Moller Plesset perturbation theory calculations.