Ab Initio Fragment Molecular Orbital-Based Molecular Dynamics (FMO-MD) Simulations of (NH3)32 Cluster: Effects of Electron Correlation

An isolated cluster of 32 ammonia (NH3) molecules was simulated at 50 and 100K by the ab initio fragment molecular orbital based molecular dynamics (FMO-MD) method to analyze the effect of electron correlation via second-order MollerPlesset perturbation (MP2). Both the energy and force of the system were calculated at the MP2 and Hartree-Fock (HF) levels with the 6-31G** basis set. The radial distribution and angular distribution functions were obtained from the trajectories. A comparison of MP2 and HF suggested that the electroncorrelation effect via MP2 leads to relative condensation through electron delocalization, similar to the case of water. Vibrational spectra were also calculated at both the MP2 and HF levels, and the former gave a more minute split of the higher modes. This again suggested that MP2 better describes the electron delocalization.