kgururangan / active_space_cc_codegen

Active-Space Coupled-Cluster Code Generator

This repository contains code to automatically generate the numerous tensor contractions arising from active-space coupled-cluster (CC) models, such as those of the CCSDt and CCSDtq types. The resulting expressions are spin-integrated. The expressions for the active-space theory of interest are formed by first taking a set of factorized spin-integrated diagrams corresponding to the parent CC theory (e.g., CCSDT or CCSDTQ), and subsequently applying all possible combinations of active/inactive partitioning to the external and contracted lines. In doing so, special attention must be paid to account for all weight and phase factors as well as the additional diagrams that arise from permuting non-equivalent indices belonging to different sectors (active or inactive) of the single-particle orbital space.