It is indeed with pleasure that we dedicate the present contribution to Professor Per Olov Löwdin in the sixtieth anniversary of his birth. From a glance at his bibliography, we could say that his interest in scattering theory has been dormant since his early days as a docent in Uppsala, time at which he lectured on the subject /1/, and led him to study Hulthén’s variational principle for the phase shifts /2/. Dormant however, does not mean inexistent: it is enough to recall the number of times he raises the question:“what about the continuum?” during seminars and conferences. In a paper with H. Shull /3/, they actually included the continuum spectrum in a configuration interaction calculation for the helium atom. This was achieved by means of a “discretization” of the continuum, with excellent results for the ground state of He.
KeywordsGreen Function Imaginary Axis Helium Atom Short Range Potential Configuration Interaction Calculation
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