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Abstract An efficient scheme for calculating one‐ and two‐electron transition density matrices for two wave functions is described. The method applies to CAS (complete active space) wave functions and certain multireference CI expansions. The orbital sets of the two wave functions are not assumed to be equal. They are transformed to a biorthonormal basis, and the corresponding transformation of the CI coefficients is carried out directly, using the one‐electron coupling coefficients.
Per‐Åke Malmqvist (Wed,) studied this question.