Hydrogen-bonded neutral clusters of ammonia, (NH3)(n) (n = 2-6), have been
theoretically investigated employing the 6-31++G(d,p) land wherever necessa
ry, higher) basis sets at the Hartree-Fock (HF) level as well as with secon
d-order Moller-Plesset (MP) perturbation theory. While the ammonia trimer a
nd tetramer exhibit perfect molecular symmetries and are non-polar, the pen
tamer and the hexamer both optimize with slight deviations from perfect sym
metries and are seen to possess marginal, but nonzero dipole moments, in co
ntradiction to a conjecture made in 1979. The (NH3)(n) 'linear' clusters ar
e seen to be chemically softer than the corresponding 'cyclic' ones. (C) 20
01 Published by Elsevier Science B.V.