Monte Carlo simulations have been performed for liquid ammonia at 277 K and 1 atm based on the pair approximation with and without adjusted analytical pair potentials. The NH3-NH3 potential function used in the first simulation has been developed on the basis of ab initio dimer calculations at the Hartree-Fock level with triple zeta plus polarization function basis sets. For the second run without the pair potential, the pair interactions have been calculated directly during the simulation using the first-principles ab initio method with the same basis sets. The nitrogen-nitrogen radial distribution function (RDF) obtained from the latter case, which is considered the best structural data based on the pair approximation, shows a first peak at 3.4 Angstrom followed by a broad shoulder ranging from 4.2 to 4.8 Angstrom. This shoulder has been observed for the first time, theoretically, in addition to that reported experimentally at 3.7 Angstrom. Furthermore, energetic error due to three-body effects has been examined. Its effects on the N-N RDF at short distances has been clearly detected. Sensitivity of the structural properties of the solution on the intermolecular (both pair and three-body) interactions has been extensively discussed.