  The theoretical treatment of mixed-crystals is very demanding. A
straight-forward approach to attack this problem is using a super cell method
(SCM). Another one is the Virtual Crystal Approximation (VCA), which is an
undocumented feature of the Vienna Ab initio Simulation Package (VASP). For
comparison we use both methods to calculate the total energy (Etot) and the
density of states (DOS) of bulk GaAsxP1-x. We then apply VCA to compute the
stability of different surfaces using an extended version of the surface
formation energy Omega. Our calculations show, on one hand, a working VCA
implementation with its flaws (overestimation of Etot) and strengths (well
modelling of DOS). On other hand, a further result is that bulk of the slab of
a mixed-crystal has a minor influence on the configuration of the surface.
