The perfect quadruples model for electron correlationin a valence active space

摘要

A local approximation to the Schrodinger equation in a valence active space is suggested based oncoupled cluster (CC) theory. Working in a pairing active space with one virtual orbital per occupiedorbital, this perfect quadruples (PQ) model is defined such that electrons are strongly correlated upto "four-at-a-time" in up to two different (occupied-virtual) electron pairs. This is a truncation of theCC theory with up to quadruple substitutions (CCSDTQ) in the active space, such that the retainedamplitudes in PQ are proportional to the fourth root of the number of CCSDTQ amplitudes. Despitethe apparently drastic nature of the PQ truncation, in the cases examined this model is a veryaccurate approximation to complete active space self-consistent field. Examples includedeformations of square H4, dissociation of two single bonds (water), a double bond (ethene), and atriple bond (nitrogen). The computational scaling of the model (fourth order with molecule size) isless than integral transformation, so relatively large systems can be addressed with improvedaccuracy relative to earlier methods such as perfect and imperfect pairing, which are truncations ofCCSD in an active space.