Ab-initio study on low-lying states of the TiSi molecule

The electronic structure of the TiSi molecule was examined using two types of multireference single and double excitation configuration Interactions w ith highly extended basis sets, one including valence correlation and the o ther including valence and core-valence correlation. A multireference coupl ed-pair approximation (MRCPA) was further applied to the latter. The calcul ations suggest a (5)Delta ground state, and the lowest excited state is (3) Pi and is only slightly (0.12 eV as estimated by MRC-PA) above the ground s tate. The spectroscopic constants of the low-lying (1)Delta, (3)Delta, (1)P i, (5)Pi, and (7)Sigma (+) states as well as the (5)Delta ground state and the (3)Pi excited states were evaluated, and we found that the molecule has only a weak a bond and that six of the eight valence electrons essentially do not contribute to the bonding. The bonding nature of TiSi in these stat es is discussed in comparison with the TiC molecule.