Theoretical study of the water pentamer

Geometry optimizations, rearrangement mechanisms, spectral intensities, and tunneling splittings are reported for the water pentamer. Two low energy degenerate rearrangements are identified for the chiral cyclic global minimum which are analogous to processes that lead to observable tunneling splittings in the water trimer. Fourteen different pathways are characterized by ab initio calculations employing basis sets up to double-zeta plus polarization (DZP) quality with subsequent reoptimization of the associated minima using the Becke exchange and the Lee–Yang–Parr correlation functionals (BLYP) with the same basis. All the pathways have been recomputed for a number of different empirical potentials, some of which reproduce the two lowest energy degenerate rearrangements quite well. However, none of the empirical potentials support all the higher energy ab initio minima. Qualitative estimates of the two tunneling splittings associated with the lowest energy pathways suggest that at least one might be observable experimentally; the associated splitting patterns and nuclear spin weights are also reported. The corresponding stationary points were finally reoptimized using DZP basis sets plus diffuse functions with the BLYP exchange-correlation functional.