The theory and a first implementation of analytic second derivatives for the second-order energy within many-body perturbation theory [MBPT(2)] based on a restricted open-shell Hartree–Fock (ROHF) reference function are presented. The applicability of the method is demonstrated by calculating harmonic frequency and infrared intensities for the 2B1 state of the CH2N molecule, which suffers from appreciable spin contamination.
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Gauß et al. (1992) studied this question.
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