All electron ab initio investigations of the electronic states of the FeC molecule

The low lying electronic states of the molecule FeC have been investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) and multi reference configuration interaction (MRCI) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic structure of the FeC molecule is interpreted as antiferromagnetic couplings of the localized angular momenta of the ions Fe+ and C resulting in a triple bond in the valence bond sense. The electronic ground state is confirmed as being 3 . The spectroscopic constants for the ground state and eleven excited states have been derived from the results of the MRCI calculations. The spectroscopic constants for the 3 ground state have been determined as re = 1.585 Å and e = 859 cm-1, and for the low-lying 1 state as re = 1.567 Å and e = 952 cm-1. The values for the ground state agree well with the available experimental data. The FeC molecule is polar with charge transfer from Fe to C. The dipole moment has been determined as 1.86 D in the 3 ground state and as 1.51 D in the 1 state. From the results of the MRCI calculations the dissociation energy, De, is determined as 2.79 eV, and D0 as 2.74 eV.