  We develop and demonstrate how to use the GUGA-based MRCISD with Core-Valence
Separation approximation (CVS) to compute the core-excited states. Firstly,
perform a normal SCF or valence MCSCF calculation to optimize the molecular
orbitals. Secondly, rotate the optimized target core orbitals and append to the
active space, form an extended CVS active space, and perform a CVS-MCSCF
calculation for core-excited states. Finally, construct the CVS-MRCI expansion
space, and perform a CVS-MRCI calculation to optimize the CI coefficients based
on the variational method. The CVS approximation with GUGA-based methods can be
implemented by flexible truncation of the Distinct Row Table (DRT). Eliminating
the valence-excited configurations from the CVS-MRCI expansion space can
prevent variational collapse in the Davidson iteration diagonalization. The
accuracy of the CVS-MRCI scheme was investigated for excitation energies and
compared with that of the CVS-MCSCF method. The results show that CVS-MRCI is
capable of reproducing well-matched vertical core excitation energies that are
consistent with experiments, by combining large basis sets and a rational
reference space. The calculation results also highlight the fact that the
dynamic correlation between electrons makes an undeniable contribution in
core-excited states.
