Knowledge Bank

FeCO is a molecule of astrophysical interest. It also served as a bench mark molecule for testing basis sets and \textit{ab initio} calculation methods.} \textbf{106}, 314, (2001). }$,$} \textbf{104}, 146, (2000).} We have previously reported molecular constants and relative energies of $X~3\Sigma -$ and $15\Sigma -$ states of FeCO, based on the two-dimensional \textit{ab initio} potential energy surfaces (PESs) determined at the MR-SDCI+$Q$+$Erel$/[Roos ANO (Fe, C, O)] and MR-ACPF+$Erel$/[Roos ANO (Fe, C, O)] levels of theory.}, 57th OSU International Symposium on Molecular Spectroscopy, RF11, 2002.} We will report here molecular properties derived from the three-dimensional PESs calculated at the level of MR-SDCI+$Q$+$Erel$/[Roos ANO (Fe, C, O)]. Calculated bond lengths $re$(Fe--C), $re$(C--O), and dipole moment (with experimental $rs$ bond lengths}, 2118 (1997).} in parentheses) are 1.722 (1.7270) {\AA}, 1.160 (1.1586) {\AA}, and 3.20 D for the $X~3\Sigma -$ state, and 1.844 {\AA}, 1.153 {\AA}, and 0.29 D for the $15\Sigma -$ state, respectively. The relative energy of $15\Sigma -$ has been calculated to be 1.27 kcal~mol$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, to be compared with the experimental value of 3.24 kcal~mol$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$.} \textbf{98}, 7730, (1993).} The general trends in low-spin/high-spin issue reported for CoH,}, \textbf{126}, 144307 (2007).} CoCN,}, \textbf{127}, 014303 (2007).} and NiCN}, (2008), in press.} are also observed for the NiCN $X~3\Sigma -$ and $15\Sigma -$ state pair.