Ab initio investigations on neutral clusters of ammonia: (NH3)n (n=2–6)

Hydrogen-bonded neutral clusters of ammonia, (NH3)n (n=2–6), have been theoretically investigated employing the 6–31++G(d,p) (and wherever necessary, higher) basis sets at the Hartree–Fock (HF) level as well as with second-order Møller–Plesset (MP) perturbation theory. While the ammonia trimer and tetramer exhibit perfect molecular symmetries and are non-polar, the pentamer and the hexamer both optimize with slight deviations from perfect symmetries and are seen to possess marginal, but nonzero dipole moments, in contradiction to a conjecture made in 1979. The (NH3)n `linearʹ clusters are seen to be chemically softer than the corresponding `cyclicʹ ones.