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C..12 `CS1`: Colle-Salvetti correlation functional

R. Colle and O. Salvetti, Theor. Chim. Acta 37, 329 (1974);
C. Lee, W. Yang and R. G. Parr, Phys. Rev. B 37, 785(1988)
`CS1` is formally identical to `CS2`, except for a reformulation in
which the terms involving are eliminated by integration by parts.
This makes the functional more economical to evaluate. In the limit of exact
quadrature, `CS1` and `CS2` are identical, but small numerical
differences appear with finite integration grids.

molpro@molpro.net 2019-03-19