  Alchemical transformations showed that perturbation theory can be applied
also to changes in the atomic nuclear charges of a molecule. The alchemical
path that connects two different chemical species involves the
conceptualization of a non-physical system in which atom possess a non-integer
nuclear charge. A correct quantum mechanical treatment of these systems is
limited by the fact that finite size atomic basis sets do not define exponents
and contraction coefficients for fractional charge atoms. This paper is
proposes a solution to this problem, and shows that a smooth interpolation of
the atomic orbital coefficients and exponents across the periodic table is a
convenient way to produce accurate alchemical predictions, even using small
size basis sets.
