Path Integral Molecular Dynamics Study on NH4+ (H2O)

Among the hydrogen bonds, which are critical to sustaining life, the interaction between ammonium ions (NH4+) and water is considered a model interaction in biological systems. Although this interaction has been examined in earlier studies on molecular dynamics, the nuclear quantum effect was not considered in those studies. In this study, we investigate the nuclear quantum effect of a singly-hydrated ammonium ion cluster, NH4+ (H2O), using an ab initio on-the-fly path integral molecular dynamics (PIMD) simulation with a BHandHLYP/6-31++G(d,p) level of calculations. We find that the nuclear quantum effect shortens the N-O bond and increases the angle of the O-N-H-b bond. The results indicate that the nuclear quantum effect has two competing contributions. The first contribution involves strengthening the intermolecular interaction, which reduces the intermolecular distance, and the other contribution involves weakening the interaction, and it facilitates the rotation of NH4+.