A molecular dynamics simulation of a water model with intramolecular degrees of freedom

Simulations of water models with and without intramolecular degrees of freedom have been performed within the classical approximation. Three H2O simulations with interaction parameters taken from a simple point charge model due to Berendsen et al. are reported: (i) with rigid water molecules, (ii) with flexible water molecules and (iii) with rigid water molecules and polarized bond lengths and bond angles obtained from the second simulation. The most important effect of the inclusion of intramolecular vibrations is seen on transport properties, which in general become a foctor of 2-3 faster. The internal vibrations lead to a small but significant lowering of the potential energy and also reduce the structure, as seen from atom-atom distribution functions. The dipole moment fluctuations are affected by the nuclear polarization caused by the intermolecular interactions. Two D2O simulation have been performed in order to assess the effects of isotope substitution, one with rigid molecules and the other with flexible.