Determination of conformational energy differences of propynlidyne isomers using the effective valence shell Hamiltonian method

We have applied the highly correlated ab initio effective valence shell Hamiltonian (H-v) method to determine the energy difference between the cyclic and linear isomers of propynlidyne (C3H). Calculations are also described for the vertical excitation energies, ionization potentials, electron affinities, dipole moments, oscillator strengths, and some harmonic vibrational frequencies, which are all determined using the third order H-v method. Computations at both the experimental and theoretically optimized geometries are used to illustrate the geometrical dependence of the computed properties. The H-v optimized geometry is obtained using a two-configurational reference function describing the two dominant resonance structures. Our third-order vertical excitation energy to the lowest excited state in the cyclic isomer, dipole moments, and ground state isomer conformational energy difference are all in good agreement with experiment and with other highly correlated many-body calculations. The computations for higher excited states and for ionization potentials, electron affinities, and oscillator strengths represent the first reports of these quantities. An explanation is provided for persistent theoretical difficulties in computing b(1) bending vibrational frequencies of the cyclic isomer. (C) 2000 American Institute of Physics. [S0021-9606(00)30321-X].