Theoretical study of low-lying excited states of HSX (X = F, Cl, Br, I)

The twelve electronic excited states of HSX (X = F, Cl, Br, I) are determined using the equation-of-motion coupled-cluster singles, doubles and with non-iterative triple excitations (EOM-CCSD(T)) method. Comparisons have been made with the two most popular multi-reference methods and detailed assignments are performed. The results show that most excitations of those species originate from the two highest occupied molecular orbitals (n → σ∗ or n → Rydberg transitions), valence like, and predominantly single excitation states. A number of Rydberg states have also been predicted. It is confirmed that the benchmark EOM-CCSD(T) results are in excellent agreement with other theoretical methods.