LOCALIZED MOLECULAR-ORBITALS IN CI CALCULATIONS OF ORGANIC-MOLECULES

Configuration Interaction (CI) calculations are carried out for bond-breaking processes by employing localized molecular orbitals (LMO). It is found that the advantage of this choice relative to the use of canonical orbitals is two-fold: First, the convergence of the truncated CI is speeded up by employing LMO; secondly, since the LMO are localized in certain regions of the molecular system one can divide the LMO in two sets, one which is important for the description of the process under consideration and the other which describes the molecular part not directly involved in the process ("spectator group"). It is found to be sufficient to correlate only the "local active region" of the molecule, and this procedure leads to a much shorter CI expansion without loss of accuracy in describing the chemical process. The CH3 group has been used as "spectator group" and the dissociation into radicals CH3X --> CH3 + X and CH3CH2X --> CH3CH2 + X with X = H, F, NH3+ as well as the charge transfer reaction CH3-X+ --> CH3+ + X and CH3CH2-X --> CH3CH2+ + X with X = N2 and NH3 has been studied. For both type of radical reactions the CH3 bonds need not be included in the local active region, while in the charge-transfer process the CH3 can only be considered a spectator group if it is not connected directly to the separating group.