Ab initio study of the lower few states of FeH: Application of the multireference coupled pair approximation

Multireference coupled pair approximation(4) [MRCPA(4)] was applied to desc ribe the ground state and the lower excited states of FeH. This study demon strates that the a (6)Delta state is 0.27 eV above the ground state, X (4)D elta, which is in good agreement with the observation (0.25 eV). The ground state is much more highly correlated than the a (6)Delta state and the use of the size-consistent method is important to predict the relative stabili ty accurately. In addition to the above results, spectroscopic data of the second (4)Delta, the lowest (4)Pi, the lowest (4)Phi, the lowest two (6)Pi, and the lowest (6)Sigma (+) states are reported. The calculated excitation energies of the lowest (6)Sigma (+) state and the second (4)Delta state ar e in good agreement with results of experiment. The total energies of the l owest (4)Delta, (4)Pi, (6)Delta, (6)Pi, and (6)Sigma (+) states are in the order of (4)Delta < (4)Pi < (6)Delta < (6)Pi < (6)Sigma (+), which supports what was anticipated previously. (C) 2001 American Institute of Physics.