CCSDTQ optimized geometry of water dimer

Abstract

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.