Seven-layer slab results for 1/4 monolayer of NH 3 on Pt(111) (so called α-NH 3 ) are compared with NH 3 on a 91-atom Pt cluster; we find that the latter closely mimics the extended surface. The calculations predict atop site occupancy for α-NH 3 with N-down. The H-bond between α-NH 3 and an additional N-down molecule (β-NH 3 ) approaching from the gas-phase is then compared with that of two molecules in the gas phase; we discover the H-bond on the surface is almost three times stronger and the bond length appreciably shorter. Geometry relaxation then results in a 65 ±5 degree tilt of the β-NH 3 axis. Finally, slab calculations with 1/4 ML each of α- and β-NH 3 support this geometry over symmetrically coordinated β-NH 3 and predict an adsorption energy in good agreement with experiment.