The VPA has been sparingly used owing to the instability of the underlying equations and the consequent difficulty of writing computer code to solve them. The absolute value of the phase shift and its variation with scattering energy is, however, needed for a full understanding of the scattering process, such as for instance in the study of shape resonances and Glory oscillations. Such methods yield the phase shift only modulo π. All other methods are based on examining the large distance behavior of the exact solution of the Schrödinger equation. This is the only exact method for the direct calculation of the scattering phase shift. The program uses the Variable Phase Approach (VPA).
The phase shift is the central quantity required for the calculation of all types of scattering cross sections. A computer code for the computation of the phase shift in atom–atom and electron–atom potential scattering is presented.
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