New implementation of the graphical unitary group approach for multireference direct configuration interaction calculations

Improved procedures were used in a new computer implementation of the graphical unitary group approach, designed specifically for efficient multireference configuration interaction calculations for molecules. These procedures include improvements in the treatment of spatial symmetry and of the multireference interacting space, and include the “repartitioned Hamiltonian,” which provides a flexible alternative to conventional particle‐hole formalisms. The new computer programs avoid the construction of very long formula files, and leave the matrix eigenvector iterations as the only significant rate‐determining step in large‐scale direct CI calculations.