The ammonia, water monomer and the trans- linear NO hydrogen- bonded cluster have been studied using ab initio calculations. The geometries of these molecules are optimized at HF, DFT, MP2, CCD and QCISD levels with several basis sets, and the interaction energies are compared to the experimental and high level computational results. The results show that, to ammonia- water cluster, calculations at the MP2/6-31 + G level bring a reasonable computational consumed cost and can give a reliable result : the errors of NH hydrogen- bond length and bond angle ^/NOH are less than 0.4 nm and 1°, respectively; the NH3'*-H20 interaction energy differs only 2.1 kj/mol from CCD and QCISD methods

Key words: AMMONIA, WATER, HYDROGEN BONDS, CLUSTER COMPOUND, AB INITIO CALCULATION, INTERACTIONS