Graphical unitary group approach based on hole-particle symmetry using in multi-reference configuration interaction

Abstract

Graphical unitary group approach (GUGA) based on the hole-particle symmetry is used to develop an efficient multi-reference configuration interaction (MRCI) algorithm. By dividing the orbital into the doubly occupied, active and external orbitals, the coupling coefficient can be calculated as the product of three segmental factors in each orbital space. Only segmental factors in the active space needs to be stored that reduces the storage requirements significantly. Therefore, MRCI calculation with more than 32 internal orbitals can be performed by our program easily. The application of hole-particle symmetry also allows us to divide the configuration space into a serial of sub-CI spaces according to the number of holes in the doubly occupied space and the number of electrons in the external space. The internal contraction of configuration space can be performed within the individual sub-CI space. Consequently, a new internally contracted MRCI (ic-MRCI) algorithm based on GUGA is proposed and developed. Benchmark calculations illustrate that the large scale MRCI calculation with several billions of un-contracted configuration state functions (CSFs) has already become a routine task.