Studies in hydrogen bonding: Association in ammonia using an empirical potential

The empirical potential EPEN/2 has been used to establish the structures of isolated hydrogen‐bonding ammonia clusters. The most stable forms of the dimer have a linear or near‐linear structure. The trimer has a closed structure with zero dipole moment. Two stable tetramer forms were found: one with a closed structure and zero dipole moment in agreement with experimental findings, and one with a pyramidal structure with nonzero dipole moment which may be an artifact of the EPEN/2 potential. The relation of the dimer structures to the limited available experimental information is discussed.