The interacting correlated fragments method with non-orthogonal orbitals

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.