Molecular Dynamics Methods in Studying Phase Equilibria

Tahir Cagin
Computer Aided Innovation of New Materials II, pp 255-259
Invited Paper presented in CAMSE'92, September 22-25, 1992, Yokohama, Japan
Eds. M. Doyama, J. Kihara, M. Tanaka, R. Yamamoto, North Holland:1993.


Two molecular dynamics methods which are useful in studying phase equilibria are presented. Both are based on the extended system Hamiltonian formalism. With the first method, Grand Canonical Molecular Dynamics (GCMD), the physical system is coupled to a chemical potential reservoir. The number of molecules or concentration of species in the physical system varies continously. In order to accomodate a continuous variation in the number of molecules, a real number variable is used. The variation in the number variable is determined from a dynamical equation of motion which depends on a prespecified chemical potential. The number of molecules is determined by taking the integer part of number variable, nu. The remainder of nu represents a fractional mlecule and is used to scale the interactions of the fractional particle with the rest of the system under study.

The second method, Gibbs Ensemble Molecular Dynamics, combines the essential features of the GCMD method with the two subsytem approach used in the Gibbs Ensemble Monte Carlo (GEMC) method. As in the GEMC, the model system is composed of two subsystems in thermal, material and if desired in mechanical equilibrium. Material equilibrium is facilitated through a transfer equation which is derived from the microscopic expression for the chemical potential of a closed system. In addition to the phenomenological foundations for these methods, examples of applications to a homogeneous simple fluid and to a molecular liquid are presented.