CEPA calculations on open-shell molecules. VI. The first ionization potential of HCO

According to an investigation by Dyke et al. (1980) HCO seems to be one of the rare cases in which the correlation energy of the neutral molecule is smaller (in absolute value) than that of the first monopositive ion, such that the ΔSCF method yields a value for the first vertical ionization potential (IP _v ) which is larger than the experimental value. In order to understand this observation we have performed a series of SCF and CEPA calculations on HCO and HCO⁺ using as many as ten different orbital basis sets. The best ΔSCF result for IP _v is 9.24 eV, i.e. slightly smaller than the “experimental electronic” IP _v of 9.38 eV. Inclusion of electron correlation lowers IP _v as long as small basis sets are used, the convergence with increasing basis size is very slow. Extrapolation to a complete basis leads to a CEPA estimate of 9.26±0.10 eV for IP _v (and 8.05 ±0.10 eV for the adiabatic IP) which shows that the correlation contribution to IP _v is indeed very small. The reason for this is that the gain in correlation energy in HCO due to the presence of the unpaired electron is compensated by a loss of core correlation energy since the low-lying antibonding in-plane C-O-π- orbital is only partially available for excitation in HCO, but fully available in HCO⁺.