Zero-field splitting tensor of the triplet excited states of aromatic molecules: A valence full-π complete active space self-consistent field study

A method to predict D tensor in the molecular frame with multiconfigurational wavefunctions in large active space was proposed, and spin properties of the lowest triplets of aromatic molecules were examined with full-π active space; such calculations were challenging because the size of active space grows exponentially with the number of π electrons. In the method, the exponential growth of the complexity is resolved by the density matrix renormalization group (DMRG) algorithm. From the D tensor, we can directly determine the direction of the magnetic axes and the ZFS parameters, D- and E-values, of the phenomenological spin Hamiltonian with their signs, which are not usually obtained in ESR experiments. The method using the DMRG-CASSCF wavefunction can give correct results even when the sign of D- and E-values are sensitive to the accuracy of the prediction of the D tensor and existing methods fail to predict the correct magnetic axes.