Multiconfigurational self-consistent field study of the excited state properties of 4-thiouracil in the gas phase

A comprehensive theoretical study of the vertical singlet and triplet electronic transitions of 4-thiouracil was performed at the multiconfigurational self-consistent field (MCSCF) level of theory. The ground state geometry of the molecule was optimized at the MP2/6-311++G(d,p) level. The MCSCF calculations were performed using the 6-311+G(d) basis set. The active space for the MCSCF calculations consisted of 12 orbitals in which 6 orbitals were the occupied pi type while the remaining 6 orbitals were the virtual pi* type. To compute the npi* transitions, the two occupied orbitals were replaced with two a orbitals localized at the thiocarbonyl and carbonyl groups, respectively. Further, MCSCF calculations were also performed with a slightly smaller active space where 10 electrons were distributed in II orbitals. The effects of dynamic electron correlation on the MCSCF energies were considered at the second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. The computed transition energies after the MCQDPT2 correlation were found to be in agreement with the experimental data. The ground and excited state molecular electrostatic potentials and the Mulliken charge distributions in different states were also investigated.