NONLINEAR TERMS DUE TO MANY-PARTICLE CORRELATION IN THE DENSITY-FUNCTIONAL THEORY

A method of calculating nonlinear terms in free energy is studied for the density functional theory of a molecular system with orientational freedoms. In the method, a free energy functional is expanded into Ta ylor series of density fields, and then the coefficients are calculate d from the cumulant expansion. Using the method, expansion coefficient s of third and forth orders are explicitly derived by the three-point and four-point coefficients of the cumulant expansion. For the third-o rder coefficient, two approximations are compared by calculations of t he triplet direct correlation function. The approximation obtained by expansion in terms of pair correlation functions gives abrupt changes in the triplet direct correlation function. The simplified Kirkwood su perposition approximation gives slow changes in the function, though i ts value should be scaled down by a small parameter. (C) 1996 American Institute of Physics.