Abinitiosearch for the equilibrium structure of the ammonia dimer

摘要

The equilibrium structure of the NH3dimer is investigated using large, efficient basis sets, 6ndash;311+G(3d,2p) and lsqb;7s5p3d,4s1prsqb; extended with bond functions, at the secondhyphen;order Moslash;llerndash;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 withCssymmetry and the other a cyclic configuration withC2hsymmetry, are examined. In a basis without bond functions, theCsstructure is found more stable. As bond functions are added, however, theC2hstructure becomes more stable. This establishes the importance of the dispersion energy which is disproportionally underestimated for theC2hstructure in a purely nucleushyphen;centered basis. The stability of theC2hstructure relative to theCsis retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equivalentCsstructures via theC2hstructure is calculated. The resulting potential curves are extraordinarily flat in a broad region around theC2hstructure but rise steeply upon approaching theCsstructure containing a nearly linear hydrogen bond, indicating that the donorndash;acceptor interchange barrier is absent in the NH3dimer. The equilibrium structure for the NH3dimer found in the present study probably has the cyclic form withC2hsymmetry.