APPLICATION OF THE EFFECTIVE VALENCE SHELL HAMILTONIAN METHOD TO ACCURATE ESTIMATION OF VALENCE AND RYDBERG STATES OSCILLATOR-STRENGTHS ANDEXCITATION-ENERGIES FOR PI-ELECTRON SYSTEMS

The ab initio effective valence shell Hamiltonian (H-v) is used to com pute the low lying vertical excitation energies and oscillator strengt hs for ethylene, trans-butadiene, benzene and cyclobutadiene. Calculat ed excitation energies and oscillator strengths of ethylene, trans-but adiene and benzene to various valence and Rydberg states are in good a greement with experiment and with values from other highly correlated computations. The present work further investigates the dependence of H-v computations on the nature and choice of the molecular orbitals an d provides a comprehensive study of the convergence with respect to th e enlargement of the valence space. Minimal valence space H-v computat ions yield very accurate estimates of the excitation energies for the low lying excited triplet states and are slightly poorer (a deviation of less than or equal to 0.5 eV from experiment) for low lying excited singlet states. More accurate low lying singlet state excitation ener gies are achieved by slightly enlarging the valence space to include R ydberg functions. The computed oscillator strengths from the H-v metho d are in excellent agreement with experiment and compare favorably wit h the best theoretical calculations. A very quick estimation of the tr ansition dipoles and oscillator strengths may be obtained from second order H-v computations. The accuracy of these calculations is almost a s good as those from the more expensive third order H-v computations a nd far superior to those from other quick methods such as the configur ation interactions singles technique. Although no experimental data ar e available for the excitation energies and oscillator strengths of cy clobutadiene, our predicted values should be quite accurate and should aid in observing its pi --> pi transitions. We also provide the firs t correlated computations of oscillator strengths for excited-->excite d singlet and tripler transitions. (C) 1997 American Institute of Phys ics.