The interacting correlated fragments (ICF) method proposed by Liu and McLea
n is employed using non-orthogonal orbital spaces for the interacting fragm
ents. This way orthogonalization schemes may be avoided and the orbitals ma
y be optimized separately at each internuclear distance. This approach is i
llustrated using a calculation on the He dimer, which is one of the first s
ystems to which ICF was applied. It is found that the use of non-orthogonal
orbitals gives a more attractive potential. Although the orbital basis inc
onsistency is avoided completely, the configuration basis inconsistency sti
ll gives rise to a considerable basis set superposition error.