The Xa Exchange Approximation

The SCF-Xa Scattered-wave Method.—An alternative to the LCAO-MO method for molecules is the Xa scattered wave or multiple-scattering Xa method, which was suggested less than 10 years ago114 and now has an extensive literative. The basic theory has been refined in several directions115 and there are several excellent reviews.116-180 The method is based on the scattered-wave method in solids181 and aims to solve the Fock equation, 

Deplus, G. Leroy, and D. Peelers, Theor. Chim. Acta, 1974, 36, 109. 

0b) interatomic region between the atomic spheres and an outer sphere  

The weaknesses of the Xa approximation itself have been considered by Slater.138 He gives reasons why the present method should be poorer for small atoms than for larger ones and offers suggestions for improvement. 

Diamond137 derives the secular equations in symmetrized form. 

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