Theoretical study of rearrangements in water dimer and trimer

The global minimum and transition states for the acceptor-tunnelling, donor-acceptor interchange and bifurcation tunnelling rearrangements of the water dimer, and the single-flip, bifurcation and concerted proton transfer processes in the water trimer have been reinvestigated. Our analysis of the tunnelling splittings and spectroscopy is based on ab initio calculations at the computational level of second-order Moller-Plesset (MP2) theory with basis sets of aug-cc-pVXZ quality (X = D, T, Q for the dimer; X = D, T for the trimer). In both water dimer and trimer, the binding energy, barrier heights, intermonomer distances, and harmonic frequencies converge smoothly as the size of the basis set increases. In the water dimer, the binding energy was evaluated as 5.09 kcal mol(1), while the activation energies are 0.52 (acceptor-tunnelling) 0.79 (donor-acceptor interchange), and 1.94 kcal mol(1) (bifurcation tunnelling) at the MP2/aug-cc-pVQZ level. In the water trimer, the binding energy was evaluated as 16.29 kcal mol(1), while the activation energies are 0.28 (single-flip), 2.34 (bifurcation), and 26.36 (proton transfer) kcal mol(1) at the MP2/aug-cc-pVTZ level.