The extended system Hamiltonian for carrying out grand canonical ensemble molecular dynamics simulations is reformulated. This new Hamiltonian includes a generalized treatment of the reference state partition function of the total chemical potential that reproduces the ideal gas behavior and various previous partitionings of ideal and excess terms. Initial calculations are performed on a system of Lennard-Jones particles near the triple point and on liquid water at room temperature. (C) 1997 American Institute of Physics. [S0021-9606(97)52641-9].