Ab initio search for the equilibrium structure of the ammonia dimer

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Abstract
The equilibrium structure of the NH3 dimer is investigated using large, efficient basis sets, 6–311+G(3d,2p) and [7s5p3d,4s1p] extended with bond functions, at the second‐order Mo/ller–Plesset perturbation approximation (MP2) and higher levels. Intermolecular energies and optimized dimer structures are obtained with the full counterpoise correction for the basis set superposition error. The stabilities of two possible equilibrium structures, one containing a nearly linear hydrogen bond with Cs symmetry and the other a cyclic configuration with C2h symmetry, are examined. In a basis without bond functions, the Cs structure is found more stable. As bond functions are added, however, the C2h structure becomes more stable. This establishes the importance of the dispersion energy which is disproportionally underestimated for the C2h structure in a purely nucleus‐centered basis. The stability of the C2h structure relative to the Cs is retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equivalent Cs structures via the C2h structure is calculated. The resulting potential curves are extraordinarily flat in a broad region around the C2h structure but rise steeply upon approaching the Cs structure containing a nearly linear hydrogen bond, indicating that the donor–acceptor interchange barrier is absent in the NH3 dimer. The equilibrium structure for the NH3 dimer found in the present study probably has the cyclic form with C2h symmetry.