Theoretical and numerical estimates of the gas-liquid critical point of a primitive model for silica.

We present a numerical evaluation of the critical point location for a primitive model for silica recently introduced by Ford et al. [J. Chem. Phys. 121, 8415 (2004)]. We complement the numerical estimate with a theoretical description of the system free energy (and related thermodynamic quantities) by solving (i) the standard parameter-free first order thermodynamic perturbation Wertheim theory and (ii) an ad hoc modeling of the temperature and density dependences of the bonding free energy, inspired by the Wertheim theory but requiring one fitting parameter alpha(rho). This parameter takes into account the correlation between adjacent bonding induced by excluded volume effects. We compare the predicted critical point location in the temperature-density plane with the "exact" numerical Monte Carlo value. The critical temperature is correctly predicted by both theoretical approaches, while only approach (ii) is able to accurately predict the critical density.