A procedure is presented for generating an intermolecular potential function from the ab initio MO calculations. Our potential function consists of the electrostatic, polarization, exchange and charge-transfer and dispersion potentials. According to the procedure, we derived the intermolecular potential function for ammonia dimer from the ab initio MO calculations with the DZP basis set at the MP2 level. The potential function well reproduced the optimized geometry and the binding energy of the dimer obtained from the ab initio calculations. The heat capacity calculated from the Monte Carlo simulation with the potential function agreed well with the experimental value, while the NN radial distribution function was in roughly accord with the experiment.