A linear response approach to second-order electronic transition intensities for multiconfigurational self-consistent field wave functions
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A new theoretical approach to two-photon transition intensities at the multiconfigurational self-consistent field (MCSCF) level of theory, is described in detail. The fundamental property of an MCSCF wave function, that it is possible to define the response equations for an excited state, is a prerequisite. The method requires solely first-order multiconfigurational response calculations, because the equations involve the response of both the initial and final state. However, the method is approximate as the coupling between the +omega and -omega parts of the linear response is disregarded. The complete active space state interaction (CASSI) method is applied in the evaluation of the involved matrix elements. To illustrate the performance and the requirements of this method, it was used to determine TP transitions in trans-1,3-butadiene and trans-stilbene.