Photoionization cross sections for H_2 in the random phase approximation with a square-integrable basis

Total photoionization cross sections for H_2 are calculated in the random phase approximation (RPA) through a numerical analytic continuation procedure applied to the polarizability for complex values of the frequency. The representation of the polarizability that is required is obtained from a discrete set of excitation energies and oscillator strengths that satisfies the Thomas-Reiche-Kuhn sum rule exactly and other energy-weighted sum rules approximately. The fact that the excitation spectrum is obtained through a solution of the RPA equations with no continuum functions added to the basis makes the method well suited for general molecular photoionization calculations. The results are compared with experiment and good agreement is found