Separated pair approximation and electron correlation in linear polyenes

A separated electron pair approximation is reformulated from a variation‐perturbation point of view. A new method is proposed for determining optimum pair functions (geminals) under the strong orthogonality condition between occupied and virtual geminals. A simple formula for correlation energies is obtained, which can be easily calculated after solving simultaneous equations built from the zeroth order occupied and virtual geminals. Numerical application is made to linear polyenes within the π electron approximation. It is shown that the geminals built from localized orbitals yield larger correlation energy than those built from canonical orbitals and the former lead to the additivity of correlation energies.