Multireference coupled electron-pair approximations to the multireference coupled cluster method. The MR-CEPA1 method

Abstract

An extension of the CEPA1 method to the multi-reference case is presented, the MR-CEPA1 approach. The method takes the variationally included terms into account as in MRDCEPA and corrects for the exclusion principle violating terms as in closed shell CEPA1. It is shown that this method yields potential energy curves that are close to those of the multi-reference coupled cluster method and parallel the full CI results quite well. The size consistency of the method is as good as the MRDCEPA method and much better than approaches that ignore the VI terms like MR-ACPF and MR-AQCC. A simpler method, where the EPV terms are not corrected on an individual basis for the doubly occupied orbitals, dubbed the multi-reference averaged CEPA1 method (MR-ACEPA), which is akin to methods previously suggested by Szalay et al., is comparable in performance.

Nonvariational Orbital Optimization Techniques for the AP1roG Wave Function

“…The theoretical modeling of the quasi-reaction path for the insertion of Be into H 2 (perpendicular C 2 v pathway) to form the linear H–Be–H molecule represents a challenging problem for single-reference methods. ,,− To insert the Be atom into the H 2 molecule, the two H atoms need to be pulled apart which leads to two nearly degenerate electronic configurations (1σ u 2 and 1σ g 2 ) for stretched internuclear H–H separations. Furthermore, the quasi-degeneracy of the 2 s and 2 p orbitals in the Be atom gives further rise to two important configurations (1 s 2 2 s 2 and the 1 s 2 2 p 2 ).…”

Multiconfiguration Self-Consistent Field and Multireference Configuration Interaction Methods and Applications

“…Recently, Ruttink et al and Ben Amor et al revisited the problem of accounting for both EPV and redundancy terms in CEPA-type approaches. Their new methods are a reconsideration of Ruttink’s idea used in their MRCEPA method and its extension by Füsti-Molnár and Szalay in the MR-AQCC-mc method .…”

“…160 Ruttink et al 176 uses CEPA(1) arguments to extend MR-CEPA, and therefore the method is called MR-CEPA1. 176 The method of Ben Amor et al 177 is called class-dressed (CD) MR-CISD and uses CEPA corrections depending on the excitation class. The new methods show some improvement over MRCEPA and MR-AQCC-mc, but they also share their feature that the analytic energy gradient cannot be formulated easily.…”

“…The theoretically most advanced methods are MR-(SC) 2 -CI by Malrieu et al 163 and SS-MRCEPA(I) by Mukherjee et al 171 since these are exact CEPA methods. In addition, MR-AQCC-mc, 160 MR-CEPA(2), 169 MR-CEPA1, 176 and CD-MR-CISD 177 are methods which account for both EPV and redundancy effects. Still, in actual calculations, MR-AQCC 142,158 and MR-ACPF 81 perform better, competing with MRCC formulations 178À180 unless higher excitations are also considered in the latter.…”

A sequential transformation approach to the internally contracted multireference coupled cluster method

“…1. 38 This model has been used extensively in the literature 6,28,[39][40][41][42][43][44] to test new multireference methods since it is a prototype for symmetry forbid- den reactions. 45 In the BeH 2 model, the reaction coordinate is parametrized by a one-dimensional variable x that falls in the range [0,4] bohrs.…”