Multireference configuration interaction calculations on the FeS molecule

High-level ab initio calculations for the A(5)Sigma(+) -X5 Delta and the a(7)Sigma(+)-X-5 Delta band systems of the FeS molecule are here reported. For such, potential energy curves are calculated using the multireference configuration interaction (MRCI) method. Subsequently, the computed interaction energies were fitted to an analytical representation and a new set of spectroscopic parameters was derived. The calculations predict a dissociation energy of 27,015 cm(-1) for the X-5 Delta, in agreement with recent experimental measurements. The similarity between X-5 Delta and A(5)Sigma(+) is considerable, showing an energy separation of only 953 cm(-1). The spin-orbit coupling (SOC) effect was considered by using the Breit-Pauli operator ((H) over cap (SO)). At the equilibrium distance, the obtained SOC constant for the ground state is A(e) (X-5 Delta) = -206.6 cm(-1). The rotationless radiative lifetimes for the A(5)Sigma(+) and a'(7)Sigma(+) excited states are in the orders of milli-and microseconds, respectively, indicating that the vibronic emissions of the A(5)Sigma(+)-X-5 Delta system are too weak to be observed. [GRAPHICS] .