SECOND-ORDER INTERMEDIATE HAMILTONIAN METHOD - PILOT APPLICATIONS TO VERTICAL EXCITATIONS IN PI-ELECTRON SYSTEMS

摘要

Second-order intermediate Hamiltonian quasi-degenerate perturbation theory is applied to calculate pi-electron vertical excitation energies of ethylene and trans-butadiene. The choice of model spaces follows the physical intuition, including the excitations within the pi-shell and leaving the sigma-orbitals doubly occupied. Model spaces are optimized using the state-average CASSCF procedure. The agreement between calculated and experimental transition energies is better than 0.1 eV. The importance of a decontracted treatment of the model-space projections of wave functions is demonstrated. A simple model study of deviations from size-consistency is reported and implications for calculations on larger molecules are briefly discussed (C) 1995 John Wiley and Sons, Inc. [References: 52]