Third-Order Generalized Van Vleck Perturbation Theory Variant of Multireference Perturbation Theory for Electron Correlation

摘要

A third-order approximation to the generalized Van Vleck perturbation theory (GVVPT3) variant of multireference perturbation theory treatment of dynamic electron correlation is described. The approximation builds upon some aspects of the increasingly widely used GVVPT2, which we introduced previously (Khait et al., J Chem Phys 2002, 117, 4133). In addition, using formal relations obtained in connection with GVVPT3, a more thorough understanding of GVVPT2 is obtained, including a key diagnostic that assesses suitability of GVVPT2 for specific tasks. Computational aspects of the implementation of GVVPT3, especially the use of a macroconfiguration description of the model space, are described. The obtained GVVPT3 method call use both complete and incomplete model spaces efficiently. We demonstrate the new method on a series of model problems that demonstrate the applicability of the suggested approach. Among other tests, we show that GVVPT3 is capable of reproducing MRCISD results to within 0.01 eV for the problematic barrier height of the metastable N-2(2+).