Rigorous method for computing photoabsorption cross sections from a basis-set expansion

We present a rigorous technique, which should be applicable to both atoms and molecules, for calculating photoabsorption cross sections using square-integrable basis functions. The technique is based on the method of complex coordinates as developed by Nuttall and co-workers. In contrast to some other L2 methods, the calculations converge directly at real energies. The method is illustrated by application to the case of atomic hydrogen.