PROPERTIES OF FLEXIBLE WATER MODELS

We have studied the influence of flexibility on liquid water properties for two different potentials of intramolecular motion. Care has first been taken to explore thoroughly the simulation conditions to obtain reliable results free from artificial influences. Grafting an intramolecular potential on top of an originally rigid model introduces minor changes in the structure of the liquid at room temperatures. Dynamic properties are sensitive to the choice of intramolecular potential, but a general trend is that flexibility slows down the molecular motion. For flexible water the dynamic retardation is caused by the increase of the dipole moment, which increases the energetic coupling to the surrounding liquid, and by the intramolecular distortion, giving rise to a larger radius of gyration compared with the equilibrium monomer value. Experimental vibrational frequencies are not accurately reproduced with either the harmonic or anharmonic intramolecular potential studied. In contrast, for a nonpolar system of propane the structural and dynamic properties of the liquid remain the same whether bonds are rigid or flexible.