The interacting correlated fragments method with non-orthogonal orbitals

The interacting correlated fragments (ICF) method proposed by Liu and McLean is employed using non-orthogonal orbital spaces for the interacting fragments. This way orthogonalization schemes may be avoided and the orbitals may be optimized separately at each internuclear distance. This approach is illustrated using a calculation on the He dimer, which is one of the first systems to which ICF was applied. It is found that the use of non-orthogonal orbitals gives a more attractive potential. Although the orbital basis inconsistency is avoided completely, the configuration basis inconsistency still gives rise to a considerable basis set superposition error.