Theoretical calculation of the photoionization cross sections in molecular systems

We present here in a format intermediate between a research‐revue paper and a student course some considerations about the calculation of the photoionization cross section for diatomic and small polyatomic molecular systems. Examples are taken from the work on diatomic molecules performed in Orsay, namely photoionization of CO, H2 and HI and the work on small polyatomic molecules, namely H2O and CH4, performed in the framework of a joint project between California Institute of Technology and several Universities in Brasil.