Studies of dispersion energy in hydrogen‐bonded systems. H2O–HOH, H2O–HF, H3N–HF, HF–HF

Dispersion energy is calculated in the systems H2O–HOH, H2O–HF, H3N–HF, and HF–HF as a function of the intermolecular separation using a variety of methods. M≂ller–Plesset perturbation theory to second and third orders is applied in conjunction with polarized basis sets of 6‐311G∗∗ type and with an extended basis set including a second set of polarization functions (DZ+2P). These results are compared to a multipole expansion of the dispersion energy, based on the Unsöld approximation, carried out to the inverse tenth power of the intermolecular distance. Pairwise evaluation is also carried out using both atom–atom and bond–bond formulations. The MP3/6‐311G∗∗ results are in generally excellent accord with the leading R−6 term of the multipole expansion. This expansion, if carried out to the R−10 term, reproduces extremely well previously reported dispersion energies calculated via variation‐perturbation theory. Little damping of the expansion is required for intermolecular distances equal to or greater than the equilibrium separation. Although the asymptotic behavior of the MP2 dispersion energy is somewhat different than that of the other methods, augmentation of the basis set by a second diffuse set of d functions leads to quite good agreement in the vicinity of the minima. Both the atom–atom and bond–bond parametrization schemes are in good qualitative agreement with the other methods tested. All approaches produce similar dependence of the dispersion energy upon the angular orientation between the two molecules involved in the H bond