Constant pressure and temperature (NPT) molecular dynamics simulations for 1080 ions (240 Na + or K + ,240 Si 4+ ,600 O 2- ) in 120 cells have been performed with the DL_POLY program, version 2.15. The MD calculations were performed for both silicates in the temperature and pressure ranges from 250 to 1000 K and 1.01325 bar (1 atom) to 100 kbar. The time step for the simulations was 1 × 10 -3 the system was equilibrated for a minimum of 100,000 time steps. The model potentials were used in this work are based on Vessal potential and contains both two- and three-body components. The interactions between ions were assumed to be purely Coulombic and the three- body component consists of bond-angle potentials for O-Si-O and O-Si-O of the kind described by Vessal et al ( a polynomial of bond angle ) and revised by Smith The accuracy of the model at 298.15 K and atmospheric pressure was checked against crystal structural data for both of these systems. Molecular dynamics (MD) method is used to investigate the behavior of the pressure-volume-temperature ( p-V-T ) relationship, lattice constant and thermal expansivity, heat capacity and density for sodium and potassium disilicate glasses at high pressures and temperatures. The isothermal sets of p-V data generated by simulation were each fitted to the three parameters from of the isothermal Parsafar and Mason equation of states(EOS).
