INCORPORATING INTRAMOLECULAR DEGREES OF FREEDOM IN SIMULATIONS OF POLARIZABLE LIQUID WATER

Liquid water properties are investigated for several models incorporating intramolecular degrees of freedom. Apart from the standard charge-charge and Lennard-Jones interactions the intermolecular potential explicitly includes many body effects arising from a molecular polarizability. The internal motion is modelled via a harmonic or an anharmonic bond potential. The flexibility introduces a nuclear polarization which results in an increase of the binding energy and small but significant rearrangements of the liquid structure compared to the rigid model. These changes depend on the type of intramolecular potential used. To more accurately model the nuclear polarization a variable charge model, parameterized from experimental dipole derivatives, is constructed. This model yields a liquid that is hydrogen bonded but with a loss of tetrahedral structure. Thus, this improved description of the intramolecular part of the single water molecule cannot adequately portray collective liquid water properties. It is found that it is necessary to further include the additional variation in polarizability and dispersion forces to achieve structural results that are comparable to a tetrahedral fluid. © 1990.