We introduce an approximate expression for the direct calculation of p
roperties of alloys in terms of the pure components. This rule can be
obtained as a particular case from the Bozzolo-Ferrante-Smith (BFS) me
thod, a recently developed semiempirical method for alloys. In particu
lar, we examine the application of this rule to several examples of th
e concentration dependence of the lattice parameter of binary and tern
ary alloys.