Effects of basis set and electron correlation on the calculated properties of the ammonia dimer

Abstract

A b 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 Mo/ller–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.