Ab initio dynamic dipole polarizabilities for O2, its photoabsorption spectrum in the Schumann-Runge region, and long-range interaction coefficients for its dimer

Perturbed states representing the response to an external electric field are obtained in the multireference averaged coupled pair functional formalism based on orbitals which are optimized in a perturbed multireference self-consistent field procedure. For each perturbing operator perturbed wave functions for several frequencies of the perturbing field are obtained simultaneously. I;rom these sets of perturbed states reduced spectra are derived which represent effective oscillator strength distributions. The broad shape of the Schumann-Runge continuum leads to a poor description of the dynamic polarizability for frequencies approaching the first pole when only vertical transitions are considered. To account for this effect the absorption spectrum in the Schumann-Runge region Is calculated and the polarizability is described up to the range of anomalous dispersion. The dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data where it is available. Significant differencies of up to 20% are observed between our anisotropic interaction coefficients and earlier estimates obtained from semiempirical anisotropic dipole oscillator strength distributions. (C) 1998 American Institute of Physics. [S0021-9606(98)30246-9].