In an MCSCF calculation, the set of coefficients of both the CSFs or determinants and the basis functions in the molecular orbitals are varied to obtain the total electronic wavefunction with the lowest possible energy.
The ground-state wavefunction for H2 at the equilibrium geometry is dominated by the configuration (φ1)2, which means that the molecular orbital φ1 is nearly doubly occupied.
At large separations, however, the terms describing both electrons located at one atom remain, which corresponds to dissociation to H+ + H−, which has a much larger energy than H· + H· (two hydrogen radicals).
In this multiconfigurational description of the H2 chemical bond, C1 = 1 and C2 = 0 close to equilibrium, and C1 will be comparable to C2 for large separations.
[2] A particularly important MCSCF approach is the complete active space SCF method (CASSCF), where the linear combination of CSFs includes all that arise from a particular number of electrons in a particular number of orbitals (also known as full-optimized reaction space (FORS-MCSCF)).