MR-CISD investigation (including extensivity corrections) of Rydberg n-3 s and n-3p states of the smallest ionized HCl-water cluster.

CONTEXT: The [H3O(H2O)3]+Cl- ion-pair is the smallest cluster of ionized HCl in water. It has been demonstrated experimentally that it can be formed in conditions compatible with the interstellar medium (ISM). It is also a precursor of another ion-pair, H3O+(MetOH)3Cl-, suggested as an important prebiotic species in the ISM. Aiming to help the detection of the former ion-pair several properties of its n-3 s and n-3p singlet and triplet Rydberg states have been studied through a highly correlated ab-initio method. The vertical excitation energies (ΔEv), oscillator strengths (f), dipole moments (μ), diradical character, participation ratio of natural transition orbitals (PRNTO), single-excitation character (Ω), charge-transfer numbers and the singlet-triplet gaps (ΔEST) have been studied at the ground state (gs) geometry. The energies of the two lowest-lying dissociation channels have also been computed. Our highest-level results for ΔEv of the n-3 s singlet states are between 6.63 and 6.65 eV, while for n-3p one has 7.40 to 7.74 eV. Previous TD-DFT results are ~ 0.84 to 0.95 eV lower, but our results nicely agree with available CASPT2 results for the n-3 s states. The brighter state is the 2nd n-3 s state, with f = 0.172 at the highest-level. While the gs is essentially a closed-shell state, consistent with the [H3O(H2O)3]+Cl- ion pair, all n-3 s and n-3p singlet states have high diradical characters, consistent with their small ΔEST values and with their nature as charge-recombination (CR) states. For most of the cases the CR characters are consistent with a decrease of μ. A possible deactivation pathway for the n-3 s states, involving their optimized geometries and a four-states conical intersection, is discussed. Our highest-level results yield the ionic channel 0.46 eV lower than the neutral channel. Ω indicates dominant single-excitation character for all Rydberg states, while PRNTO indicates multi-configurational character only for few n-3p states. METHODS: The multi-reference configuration interaction with singles and doubles (MR-CISD) calculations (including size-extensivity corrections) have been employed. Size-extensivity corrections have only been applied for ΔEv and for the energies of the dissociation channels. f, μ and ΔEST have been computed from the MR-CISD wavefunctions. PRNTO, Ω and the charge-transfer numbers (from which the nature of the excited states in terms of local, charge-transfer or CR states are obtained) have been computed from the one-electron transition density matrix. Three basis sets have been employed. The smallest one is the Pople's 6-31+ +G**, while the second one is a mixed basis set based on the aug-cc-pVDZ and the d'-aug-cc-pVDZ (a reduced d-aug-cc-pVDZ basis set). The third one is also a mixed basis set, consisting of the aug-cc-pVDZ and the universal Rydberg basis set of Kaufman et al. The possible deactivation pathway for the n-3 s states has only been studied at the MR-CISD level with the smallest basis set.