On accuracy of Wertheim's thermodynamic perturbation theory for primitive models of water

Applicability and accuracy of Wertheim's thermodynamic perturbation theory (TPT) is examined in detail using three qualitatively different primitive models of water. Theoretical results for the thermodynamic properties of these models obtained using the theory in its first order (TPT1) are presented and compared with new simulation data. Individual approximations used in the implementation of the TPT1 to get close analytic results are also assessed by evaluating the appropriate quantities by computer simulations. It is shown that an approximate description of the reference system affects the overall performance of the TPT1 only marginally, if at all, and that the observed discrepancies between theory and experiment for all models are rather inherent to the theory itself and result from the neglect of higher order terms (i.e. certain graphs) contributing to the excess Helmholtz free energy.