Balance in interaction energy calculations

Abstract
Using the interacting correlated fragments method applied to He2, we show that in order to calculate reliable interaction energies, a supermolecular calculation must be balanced both in orbital and in configuration space. The role of the Pauli exclusion principle is discussed and it is used to identify all configurations to be included in the monomer calculations. Only if the orbitals in dimer and monomer calculations are exactly matched and the monomer calculations include all configurations that formally may be considered to be present in the dimer, are the results obtained consistent with previously determined multi-reference CI and perturbation-theory estimates in all three basis sets considered.