Molecular electric properties of liquid water calculated using the combined coupled cluster/molecular mechanics method

In this paper we employ a recently developed method which combines coupled cluster and molecular mechanics (CC/MM) theories to study the effects of solvation on some molecular properties of liquid water. We focus on the effect of varying the parameters used to define the classically treated molecules, i.e. partial charges and point dipole polarizabilities. Furthermore, we define a set of 'polarization charges' which include the effects of a polarizable environment (the MM system) and correctly reproduces the ground state energy of the total system. However, we find that for these polarization charges that specific ground and excited state molecular properties are not reproduced correctly. Specially, the polarizability is very difficult to model using a simple point charge model with charges confined to the MM atoms. Furthermore, we study some excited state properties such as electronic excitation energies and corresponding dipole transition moments of liquid water. Finally, we explore the effect of introducing few or many classically treated molecules in the CC/MM calculations. (C) 2003 Elsevier B.V. All rights reserved.