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

The order-disorder transition of Cu3Au is studied using the constrained and unconstrained (constant temperature and pressure) method. Two constrains are applied, namely the constant volume and the fixed atomic positions. A N-body potential is utilized determined from the zero temperature structural properties of fcc Au, Cu and Cu3Au. The transition temperature determined using the present potential and the constrained Monte-Carlo method agrees well with the experimental one. Releasing the constant volume constrain we obtain a charge in the lattice constant at the transition temperature which is twice as large as the experimental data indicate. The unconstrained Monte-Carlo method predicts very well the change of the lattice constant and the mean-square displacement at the order-disorder transition. Only the transition temperature 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.