The effect of a variety of theoretical methods (HF, B3-LYP, MP2, QCISD, and CCSD(T)) and basis sets (from 6-31G(d) to 6-311+G(3df,2p)) on the calculated geometry and dimerization energy of the acetylene-ammonia dimer HCCH...NH3 is examined. The dimer has C-3t symmetry with acetylene acting as the hydrogen bond donor. Our highest level calculations (viz. CCSD(T)/6-311+G(3df,2p) including BSSE correction) predict an equilibrium contact distance r(H...N) of 2.280 Angstrom and an equilibrium binding energy Delta E-c of 14.1 kJ mol(-1). incorporation of a scaled B3-LYP/6-311+G(3df,2p) zero-point vibrational correction lends to Delta E-0 = 9.3 kJ mol(-1).The less expensive CCSD(T)/6-311+G(3df,2p)//B3-LYP/6-311+G(3df,2p) procedure reproduces these benchmark energies and is therefore recommended for general application on small hydrogen-bonded systems. For larger hydrogen-bonded systems, the still less expensive B3-LYP/6-311+G(3df,2p)//B3-LYP/6-311+G-(d,p) procedure is recommended, and this yields Delta E-0 = 7.8 kJ mol(-1) for the acetylene-ammonia dimer.