Triplet excitation properties in large scale multiconfiguration linear response calculations

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Abstract
It is shown that large scale MCSCF linear response (MCLR) calculations can be carried out efficiently when the perturbation operator has triplet symmetry. Methods have been developed to efficiently construct two electron density matrices containing triplet orbital excitation operators and two electron density matrices which connect singlet and triplet states. Direct CI methods have also been developed for two electron operators with spin rank one. Numerical calculations are presented for CH+ and the results compared with full CI results.