Eclectic Electron-Correlation Methods

An eclectic combination of cluster, perturbation, and linear expansions often provides the most compact mathematical descriptions of molecular electronic wave functions. A general theory is introduced to define a hierarchy of systematic electron-correlation approximations that use two or three of these expansion types. It encompasses coupled-cluster and equation-of-motion coupled-cluster methods and generates various perturbation corrections thereto, which, in some instances, reduce to the standard many-body perturbation methods. Some of these methods are also equipped with the ability to use basis functions of interelectronic distances via the so-called R12 and F12 schemes. Two computer algebraic techniques are devised to dramatically expedite implementation, verification, and validation of these complex electron-correlation methods. Numerical assessments support the unmatched utility of the proposed approximations for a range of molecular problems.