RELATIVE STABILITY OF THE (3)A(2), (1)A(2), AND (1)A(1) STATES OF PHENYLNITRENE - A DIFFERENCE-DEDICATED CONFIGURATION-INTERACTION CALCULATION

The relative stability of the (3)A(2), (1)A(2), and (1)A(1) states of phenylnitrene is evaluated by means of ab initio calculations followed by difference-dedicated configuration interaction (DDCI). This approa ch is based on effective Hamiltonian theory at a low order of perturba tion to select rationally the determinants which contribute to the ene rgy difference. The CI space built on this criterion is then treated v ariationally. The method allows a considerable reduction of the CI spa ce compared with a complete CASSDCI calculation (where CAS stands for complete active space). Depending on the concerned ene:gy difference, different model spaces may be chosen, as illustrated in the (3)A(2)-- >(1)A(2) and the (3)A(2)-->(1)A(1) transitions in phenylnitrene. Since the CI space may reach considerable dimensions, a direct CI algorithm for selected CI spaces, the SCIEL algorithm, has been used to perform the calculations. The results are in excellent agreement with previou s calculations and with available experimental data. (C) 1996 by John Wiley & Sons, Inc.