New schemes for internally contracted multi-reference configuration interaction

In this work we present a new internally contracted multi-reference configuration interaction (MRCI) scheme by applying the graphical unitary group approach and the hole-particle symmetry. The latter allows a Distinct Row Table (DRT) to split into a number of sub-DRTs in the active space. In the new scheme a contraction is defined as a linear combination of arcs within a sub-DRT, and connected to the head and tail of the DRT through up-steps and down-steps to generate internally contracted configuration functions. The new scheme deals with the closed-shell (hole) orbitals and external orbitals in the same manner and thus greatly simplifies calculations of coupling coefficients and CI matrix elements. As a result, the number of internal orbitals is no longer a bottleneck of MRCI calculations. The validity and efficiency of the new ic-MRCI code are tested by comparing with the corresponding WK code of the MOLPRO package. The energies obtained from the two codes are essentially identical, and the computation...