Size consistency in multi-reference double excitation CI calculations

The problem of size consistency in a multi-reference CI calculation is discussed using the method of quasi-degenerate many-body perturbation theory suggested by Brandow [1, 2]. The multi-reference form of linear coupled pair many-electron theory (CPMET), the coupled electron pair approximation (CEPA) and the Davidson correction are derived. It is shown that these methods are easily implemented in quantum chemical calculations.