On the stability of the cuboid singlet (S2)4 supermolecule: benchmark ab initio studies

We report high level ab initio supermolecular calculations for the cuboid structure of the disulfur tetramer, (S2)4. Accurate geometries and interaction energies with respect to 4S2 ((3)Σg (-)) were obtained using four different methods, Möller-Plesset perturbation theory (MP2), complete-active-space SCF (CASSCF) + complete active space second-order perturbation (CASPT2), RCCSD(T), and a hybrid CASPT2(singlet-nonet)∕RCCSD(T)-nonet approach with systematic sequences of augmented correlation-consistent basis sets extrapolated to the complete basis set limit. Unlike the van der Waals-like (O2)4 cluster, (S2)4 is found to be much more chemically bound. Our best estimate of the dissociation energy to four S2 molecules is 65 kcal∕mol including the counterpoise correction and an intermolecular distance of 2.74 Å. The singlet ground state of (S2)4 is much less multiconfigurational than that of (O2)4 van der Waals complex, which allows a reliable CCSD(T) description of the singlet potential energy surface of the supermolecule around its equilibrium geometry. The electron pair localization function clearly reveals electron pairing between the S2 units in the complex at the ROHF and the CASSCF∕aug-cc-pVTZ levels. Vibrational analysis at the MP2∕cc-pV(D,T,Q)Z,aug-cc-pVTZ levels yield stable cuboid structures; however, at the CCSD∕aug-cc-pV(D,T)Z levels this analysis reveals a transition state with one imaginary frequency. Thus, further multireference-based studies with large basis sets are required to reliably settle the stability issue for this supermolecular sulfur species.