A pseudopotential approach has been used to study the variation of the vacancy formation energy as a function of the parameter of Ashcroft's empty core model potential (AECMP) and nine different exchange and correlation functions (ECF) for some trivalent fcc metals, viz. Al, La, Sc, γ-Ce, Pr and Yb. The criterion used is that should be greater than the Bohr radius ( ). There is a systematic increase in the fitted value of in going from one ECF to another, as follows: KK < Sham < GV < Kle < Harr < VS < Tay < Hub < MD and it is difficult to assign a particular value of for all ECFs for a particular trivalent fcc metal. The inherent simplicity of AECMP makes it difficult to have a universal parameter for all types of atomic property calculations.