Calculation of transition moments between internally contracted MRCI wave functions with non-orthogonal orbitals

We present an efficient algorithm for calculating transition properties between internally contracted multireference configuration interaction (MRCI) wave functions using different orbitals for both states. The method is applied to low lying electronic states of aluminium monoxide, and transition moment functions computed with the same or different orbital sets for the individual states are compared. The difference between transition moments obtained with state-averaged or state-specific orbitals is found to be rather small at the MRCI level, but substantial at the CASSCF level.