Grand canonical molecular dynamics for TIP4P water systems

Abstract

An algorithm was developed enabling implementation of a Nosé—Hoover thermostat within the framework of grand canonical molecular dynamics [Lynch, C. G. and Pettitt, B. M., 1997, J. chem. Phys., 107, 8594]. The proposed algorithm could readily be extended to mixtures of molecular species with different chemical potentials as shown in the paper. This algorithm was first applied to simulate a μVT ensemble of TIP4P water molecules at 298 K by means of a system comprising a number of full particles and a single scaled (fractional) particle, with the scaling factor considered as a dynamic variable in its own right and chemical potential a pre-set parameter. Our finding showed that the scheme with a single fractional particle tended to freeze in metastable states as well as failed to reproduce either the real-life (−24.05 kJmol⁻¹) or the model-specific chemical potential of water (−23.0kJ mol⁻¹). In order to overcome the inadequacy of a single fractional particle as a chemical potential ‘probe’ the treatment of Pettitt and co-workers was extended to introduce multiple fractional particles. The extended scheme (with 4 fractional particles) was able to reproduce the actual density of water for the driving chemical potential of -24.0k mo⁻¹. The actual behaviour of the density as a function of the chemical potential also agreed quite well with both the results of thermo-dynamic integration and the findings of Pettitt and co-workers.