Ab initio calculations of spin–orbit contribution to the zero-field splitting tensors of nπ∗ excited states by the CASSCF method with MRMP2 energy correction

The spin–orbit contributions (DSO tensors) to the zero-field splitting tensors (D tensors) in the excited triplet states of CS2 and diazabenzenes are investigated by using the sum-over-states formula with the CASSCF spin–orbit coupling matrices and the CASSCF or MRMP2 energies. The DSO tensor calculated using the CASSCF energies is in some cases far from the experiment due to inaccurate energy differences, but the discrepancy can be circumvented by replacing the CASSCF energies with the MRMP2 ones. Effects of the geometry relaxation in the excited states on the D tensors are also discussed.