A many‐body perturbation theory and coupled cluster study of the water dimer

The interaction energy of two water molecules is evaluated at several levels of theory in a better than double zeta plus polarization basis set. Our CCSD+T(CCSD) results should be quite close to the full CI result in this case. Results are compared with variation–perturbation results for the dispersion energy. Counterpoise‐corrected correlated results are used to examine the importance of basis set superposition effects in this system.