Basis set limit geometries for ammonia at the SCF and MP2 levels of theory

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Abstract
A systematic approach to the Hartree–Fock limit geometry of ammonia was used to determine the limiting bond length, r(NH)=0.998±0.001 Å, and the limiting bond angle (HNH) =108.1±0.1°. Multiple sets of polarization functions, d functions on hydrogen, and f functions on nitrogen are all needed to converge the bond angle to within ±0.1° of the limiting value. Geometry changes due to MP2 correlation mostly become smaller as the basis set increases in size. The MP2 structures are not as well converged to a basis set limit as the SCF structures: r(NH)=1.007±0.002 Å and (HNH)=107.5±0.4°.