STRUCTURE OF THE AMMONIA DIMER STUDIED BY DENSITY-FUNCTIONAL THEORY

Self-consistent Kohn-Sham density functional calculations have been carried out to study the structure of the ammonia dimer. The local-density approximation yields unusually large binding energy and short internitrogen distance compared with the experimental and more accurate theoretical data. The results from the Becke-Perdew gradient-corrected functionals are generally in good agreement with those at the SCF MP2 level when the geometry is fully optimized with various large basis sets. With our best estimation, the staggered quasi-linear structure (C-s) is 0.6 kcal/mol lower in energy than the symmetric cyclic one (C-2h) The hydrogen-bonded N-H bond in the staggered quasi-linear structure is found to be 0.008 Angstrom longer than the N-H bond in ammonia. In our calculations, we could not find the minima on the energy surface corresponding to the two asymmetric cyclic structures suggested by microwave spectra and coupled pair functional calculations. (C) 1994 John Wiley & Sons, Inc.