Ab Initio MRCI+Q Study on Low-Lying States of CS Including Spin–Orbit Coupling

Carbon monosulfide (CS), which plays
an important role in a variety
of research fields, has long received considerable interest. Due to
its transient nature and large state density, the electronic states
of CS have not been well understood, especially the interactions between
different states. In this paper, we performed a detail ab initio study
on the low-lying electronic states of CS by means of the internally
contracted multireference configuration interaction method (including
Davidson correction) with scalar relativistic correction using the
Douglas–Kroll–Hess Hamiltonian. We focused on the spin–orbit
coupling of the states via the state interaction method with the full
Breit–Pauli Hamiltonian. The potential energy curves (PECs)
of 18 Λ–S states correlated with the lowest dissociation
limit of the CS molecule were calculated, as well as those of 50 Ω
states generated from the Λ–S states. The spectroscopic
constants of the bound states were obtained, which are in good agreement
with previous available experimental and theoretical results. The
state perturbations of the a3Π and A1Π
states with other low-lying electronic states are discussed in detail,
based on the calculated spin–orbit matrix as well as the PECs
of the Ω states. Avoided crossing in the states of CS was indicated
when spin–orbit coupling was taken into account. Finally, the
allowed transition dipole moments as well as the lifetimes of the
five lowest vibrational states of the A1Π1, A′1Σ+0+ and a3Πi states were obtained.