A multireference configuration interaction (MR-CI) based method (Spectroscopy ORiented CI, SORCI) is proposed to calculate energy differences between several electronic states of possibly different multiplicity and which can be extended to fairly large molecules. It combines several well-known concepts in order to achieve efficiency, balance, generality, stability and accuracy: (a) the use of any kind of (preferably small) reference space and any kind of initial self-consistent field treatment, (b) the use of average approximate natural orbitals (AANOs) for the states of interest, (c) a variational treatment of the strongly interacting many electron states combined with second order multireference Møller–Plesset perturbation theory for the much larger weakly interacting remainder, (d) the concept of difference-dedicated CI developed and extensively used by Malrieu and co-workers that avoids contributions which are expected to cancel in energy differences, (e) the use of the resolution-of-the identity approximation to avoid exceedingly large integral files. The method is completely devoid of intruder state problems and makes extensive use of truncation techniques in order to achieve computational efficiency. Currently three cutoffs are introduced that control the size of the reference space, the size of the variational treatment and the size of the AANO basis. Test calculations show that the method is stable against variations of these parameters over a wide range. A study of prototypical molecules ranging in size between diatomics and free-base porphin indicates that the method yields accurate results even with moderate basis sets and CI dimensions that typically do not exceed ∼105 configuration state functions. Transition energies are typically predicted within 0.1–0.3 eV from the experimental values.
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