Second‐order intermediate Hamiltonian method: Pilot applications to vertical excitations in π‐electron systems

Second‐order intermediate Hamiltonian quasi‐degenerate perturbation theory is applied to calculate π‐electron vertical excitation energies of ethylene and trans ‐butadiene. The choice of model spaces follows the physical intuition, including the excitations within the π‐shell and leaving the σ‐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. © 1995 John Wiley & Sons, Inc.