Theoretical study of low-lying electronic states of TiCl and ZrCl

Low-lying electronic states of TiCl and ZrCl were investigated by the complete active space SCF (CASSCF), multi-reference singly and doubly excited configuration interaction (MRSDCI), and multi-reference coupled pair approximation (MRCPA) calculations using the model core potential (MCP) method. Relativistic effects were incorporated in the MCP and basis sets for Zr at the level of Cowan and Griffin's quasi-relativistic Hartree-Fock method. The (4)Phi state was found to be the ground state of TiCl, whereas the (2)Delta state was the ground state of ZrCl at all levels of calculation. Two low-lying excited states were very close in energy to the ground state. The excited (4)Sigma(-) and (2)Delta states of TiCl were higher than the ground state by 0.102 eV and 0.458 eV, respectively, and the excited (4)Phi and (4)Sigma(-) states of ZrCl were higher by 0.094 eV and 0.110 eV, respectively, at the MRCPA level. The calculated values of r(e)(2.319 A) and omega(e)(382 cm(-1)) for the ground (4)Phi state of TiCl are quite close to the values of r(e)(2.351 A) and omega(e)(383 cm(-1)) for the ground (2)Delta state of ZrCl. The values of r(e), omega(e), B-e, and alpha(e) are reasonably comparable to the observed values for both TiCl and ZrCl. (C) 1999 American Institute of Physics. [S0021-9606(99)31033-3].