Knowledge Bank

FeCO has been used as a benchmark molecule to evaluate the basis functions for $Fea,b$ and the methods of calculation since it is known to be difficult to reproduce the energy difference between the $a~5\Sigma -$ and the $X~3\Sigma -$ states as well as the experimentally observed bond lengths of the $X~3\Sigma -$ state by ab initio molecular orbital calculations. We carried out the MR-SDCI + Q and MR-ACPF calculations, based on the state-averaged MCSCF orbitals, taking into account the electron-correlation of $8-10\sigma$ electrons with the active space consisting of Fe 3d, 4s orbitals and $CO\pi ,\pi \ast$ orbitals. Our predicted term value of the $a~5\Sigma -$ state, bond lengths $re(Fe-C)$ and $re(CO)$ of the $X~3\Sigma -$ state are 0.87 kcal $mol-1,1.720\backslash AA$, and $1.159\backslash AA$ with relativistic energy corrections, which are to be compared with the corresponding experimental values of 3.24 kcal and $1.1586\backslash AA[rs(CO)],d$ respectively. Similar results have also been obtained by the MR-ACPF methods.