Journal of Chemical Physics
Ab initio calculations are carried out for (NH3)2 with a 6‐31G∗∗(1p,2d) basis set containing diffuse polarization functions. Electron correlation is included via second‐order Møller–Plesset perturbation theory (MP2). At the SCF level, the equilibrium R(NN) distance is 3.54 Å and the interaction energy is −2.35 kcal/mol. Inclusion of correlation enhances the attraction substantially, increasing the energy to −4.05 kcal/mol and reducing the intermolecular separation by 0.20 Å. Comparison with previous results at the SCF level demonstrates a variety of errors including exaggerated dipole moments, underestimation of polarization energy, and sizable superposition errors with these smaller basis sets.
Effects of basis set and electron correlation on the calculated properties of the ammonia dimer Z. Latajka and Steve Scheiner, J. Chem. Phys. 81, 407 (1984), DOI:10.1063/1.447319