COMPARISON OF THE CONSTRAINED AND UNCONSTRAINED MONTE-CARLO METHOD - THE CASE OF CU3AU

The order-disorder transition of Cu3Au is studied using the constraine d and unconstrained (constant temperature and pressure) method. Two co nstrains are applied, namely the constant volume and the fixed atomic positions. A N-body potential is utilized determined from the zero tem perature structural properties of fcc Au, Cu and Cu3Au. The transition temperature determined using the present potential and the constraine d Monte-Carlo method agrees well with the experimental one. Releasing the constant volume constrain we obtain a charge in the lattice consta nt at the transition temperature which is twice as large as the experi mental data indicate. The unconstrained Monte-Carlo method predicts ve ry well the change of the lattice constant and the mean-square displac ement at the order-disorder transition. Only the transition temperatur e is found to be 25 % lower. The results indicate that the relaxation effects and the chemical effects are properly accounted by the N-body potential.