The density matrix in may-electron quantum mechanics III. Generalized product functions for beryllium and four-electron ions

The theory of generalized product functions is extended to non-orthogonal group functions in the case of two groups. Calculations on beryllium and on some four-electron ions are based on such group functions, and are compared with similar calculations using orthogonal group functions. I t is shown that, in the case of the systems examined, the strong orthogonality restriction is not severe.