MULTICONFIGURATION LINEAR-RESPONSE APPROACHES TO THE CALCULATION OF ABSOLUTE PHOTOIONIZATION CROSS-SECTIONS - HF, H2O, AND NE

We describe multiconfiguration linear-response (MCLR) approaches to th e calculation of absolute photoionization cross sections. Three algori thms are described: (i) one in which the MCLR equations are used to de rive the primitive spectrum of excitation energies and oscillator stre ngths; (ii) one in which the solutions of the MCLR equations are used to provide the even negative moments of the photoexcitation-photoioniz ation spectrum; and (iii) one in which a pseudospectrum is obtained di rectly in the iterative procedure used to solve the MCLR equations. Ei ther the primitive spectra (i), the moments (ii), or the pseudospectra (iii) are used as basic quantities in Stieltjes imaging to obtain the photoionization cross sections. Numerical demonstrations with large m ulticonfigurational self-consistent field reference spaces are given f or the photoionization of HF, H2O, and Ne. Comparative calculations ar e performed in the random-phase approximation. Results are analyzed wi th respect to the fulfillment of gauge invariance, sum rules, basis-se t completeness, and choice of correlating orbital spaces. Results for absolute photoionization cross sections from the MCLR algorithms agree very well with Stieltjes-imaging cross sections obtained from the sem iempirically determined spectral moments of Meath and co-workers [Can. J. Phys. 55, 2080 (1977); 63, 1616 (1985)], and distinguish the raw e xperimental cross sections.