VARIATIONAL CALCULATION OF SMALL ENERGY DIFFERENCES - THE SINGLET-TRIPLET GAP IN [CU2CL6]2-

Based on the theory of effective Hamiltonians, a variational procedure to calculate singlet-triplet energy differences in diradical systems is described, which includes all the differential second-order contributions. When applied to [Cu2Cl6]2-, for which the singlet-triplet gap ranges between -40 and 80 cm-1, depending on the structural parameters, a good agreement is reached. For large systems. a truncation procedure of the MO set based on singlet-triplet gap-dedicated MOs is described, which allows the dimension of the molecular integrals file to be reduced significantly without loss of efficiency.