Applicability of size-consistent self-consistent configuration interaction to excited states

The size-consistent self-consistent configuration interaction (SC)2CI method is applied to the calculation of excitation energies. This method diagonalizes a dressed SDCI matrix built for the ground state but it is shown that the extra eigenvalues are not phony solutions but represent the excited roots of the system. Numerical tests are encouraging. The method is further improved by using the (SC)2MRCI scheme where the desired excited determinant is added to the ground state determinant as a generator of the CI space. Excellent results are obtained with this method for single and double replacement excitations in CH2.