Ab initio molecular orbital wavefunctions have been constructed for the planar and pyramidal conformations of ammonia in its ground electronic state. These solutions are very close to the Hartree-Fock limit and exhibit a lower total energy (ET equil=-56.22191 hartree) than any other such calculations which have been carried out. The computed inversion barrier is 5.08 kcal/mole (exptl = 5.8 kcal/mole) and results almost solely from the differential mixing of d-type polarization functions in the planar and pyramidal geometries. In contrast to the conclusions of much earlier work, the inversion barrier may be quantitatively obtained within the framework of the molecular orbital (Hartree-Fock) approximation. Other properties of ammonia that have been determined and discussed are: binding energy, heat of formation, correlation energy, dipole moment, population and energy component analysis, force constants, and the Walsh diagram.