[5dyz], and 1a 2 [5dxy] in symmetry. All valence electrons were correlated in the subsequent MRCI calculation. A full MRCI geometry optimization was performed only for the ground triplet and lowest-lying quintet electronic states of HfCO. Davidson-corrected MRCI (MRCI+Q) energies obtained at the ground state MRCI geometry were also used to compute the dissociation energy (De) and the excitation energies (Te) of the HfCO. At the MRCI level spin-orbit coupling effects were evaluated by a single point calculation of the MRCI ground state geometry using the Breit-Pauli Hamiltonian as implemented in MOLPRO.