The convergence of the first-order perturbative approach was tested for the 3 n* T1 minimum geometry of BA (vertical emission) by varying the number of states used in Eq. ( ). The results for the oscillator strength [Eq. ( )] and vertical phosphorescence lifetime [Eq. ( )] are presented in Table , indicating a reasonable convergence is achieved for 20 triplet and 20 singlet states. 4 Results