Adiabatic switching applied to realistic crystalline solids: Vacancy-formation free energy in copper

We study the application of the adiabatic switching molecular dynamics method to determine bulk and vacancy-formation Gibbs free energies as a function of temperature at zero pressure for copper. The bulk free energy has been determined through isochoric isothermal switching procedures in which a system consisting of 500 copper atoms interacting through a semiempirical tight-binding potential is turned into a system of 500 independent identical three-dimensional harmonic oscillators. The equilibrium volumes of these simulations were determined from equilibrium isobaric isothermal molecular dynamics simulations. The frequency of the oscillators is chosen to be of the order of a principal phonon frequency of copper in order to achieve competitive convergence. The resulting bulk free energy and entropy are in excellent agreement with experimental values. The vacancy-formation Gibbs free energy has been computed from isobaric isothermal switching procedures in which the interactions of a single copper atom are switched off. Considering the limited accuracy of the interatomic potential and the numerical noise present in the small energy differences measured, the estimated formation enthalpies and entropies agree remarkably well with experimental data. The method has shown to be computationally efficient. Typically, 6 h of CPU time on a Digital Alpha 3000/900 were required per data point for the bulk as well as the vacancy-formation parameters.