For H2O taken as a model, a comparison is made between three approaches generally used for the determination of photoionization cross-section profile. The wave function results from an ab initio multiconfigurational calculation. The plane wave (PW) model including ionic core correlation leads to a good interpretation of the photoionization spectra and energy photon dependence is analyzed. In a L2 approach associated with the Stieltjes moment method, a comparative study of two basis sets leads us to appreciate the respective importance of Rydberg and polarization functions. The last point of our work is devoted to the calculation of photoionization cross-section from the inversion of the Kramers–Krönig dispersion integral in a gauge invariant model. This approach allows to underline the importance of the pseudo states in the continuum discretization.