2ND-ORDER INTERMEDIATE HAMILTONIAN METHOD - PILOT APPLICATIONS TO VERTICAL EXCITATIONS IN PI-ELECTRON SYSTEMS

Second-order intermediate Hamiltonian quasi-degenerate perturbation th eory is applied to calculate pi-electron vertical excitation energies of ethylene and trans-butadiene. The choice of model spaces follows th e physical intuition, including the excitations within the pi-shell an d leaving the sigma-orbitals doubly occupied. Model spaces are optimiz ed using the state-average CASSCF procedure. The agreement between cal culated and experimental transition energies is better than 0.1 eV. Th e importance of a decontracted treatment of the model-space projection s of wave functions is demonstrated. A simple model study of deviation s from size-consistency is reported and implications for calculations on larger molecules are briefly discussed (C) 1995 John Wiley and Sons , Inc.