CCSDTQ Optimized Geometry of Water Dimer.

The equilibrium geometry of the lowest energy structure of water dimer [(H2O)2] has been investigated using coupled cluster theory. A hierarchy of conventional coupled cluster methods is utilized up to singles doubles triples and quadruples excitations (CCSDTQ). The geometry of (H2O)2 is also optimized using the explicitly correlated coupled cluster singles doubles and perturbative triples [CCSD(T)-F12b] method. Overall, we find that the effect of including excitations beyond CCSD(T) is smaller than inclusion of core-valence correlation and comparable to scalar-relativistic and adiabatic effects.