Since the objective is to calculate the deexcitation energy from the relaxed 3 T 1u to the 1 A 1g state, we use TDDFT forces to obtain the optimized excited 3 T 1u geometry. We refer to the work by Jin et al. for the implementation of the TDDFT method and the details of the TDDFT calculations on MgO. With the optimized 3 T 1u and 1 A 1g geometries at hand, the TDDFT excitation energies between the two states at both geometries are computed.