MOLECULAR-DYNAMICS SIMULATIONS OF THE EFFECTS OF TRUNCATION OF THE TAYLOR EXPANSION OF THE POTENTIAL-ENERGY ON THE THERMODYNAMIC PROPERTIESOF A CRYSTAL

Molecular dynamics simulations are carried out on a Lennard-Jones crys tal, for the potential energy surfaces generated from the full Hamilto nian and the Taylor expansion of the potential energy truncated at the quartic term, to determine the accuracy of the quartic truncation wit h regard to the thermodynamic properties of a crystal. The results sho w that the errors arising from the quartic truncation become significa nt only for temperatures T> 0.2T(m), and are only on the order of 5% a t T = 0.8T(m), where T-m is the melting temperature. The quartic trunc ation represents a significant improvement over the quadratic (harmoni c) truncation, and the errors associated with the quadratic truncation are decreased by 75%. The sources of error in the lambda(2) perturbat ion theory are investigated; the errors are found to arise from the tr uncation of the potential energy expansion at low temperatures, and pr imarily from the truncation of the perturbation expansion at high temp eratures. (C) 1996 American Institute of Physics.