The computer program IMPETUS calculates sputter depth profiles by solving Fokker-Plank type equations for mixing. In the standard model, the effective volume of the mixing constituents is taken to be constant throughout the mixing regardless of the composition of their atomic environment. Experimentally measured values for atomic densities of materials and compounds indicate that this assumption is not correct and that nonconstant atomic volumes need to be considered. Although the way in which the variation of the volumes take place is not known, the present work is aimed at developing the treatment of Fokker-Plank type equations such that composition dependent effective atomic volumes can be included. The complexity of the equations and a method for solution together with its application for a simple case are explained in this paper.