Quantum Effects on Vibrational and Electronic Spectra of Hydrazine Studied by On-the-Fly ab Initio Ring Polymer Molecular Dynamics

Structural and spectroscopic properties of hydrazine, H2N-NH (2), it being a floppy or fluxional molecule in a vacuum, are investigated by means of ab initio molecular dynamics, ab initio path integral molecular dynamics, and ab initio ring polymer molecular dynamics simulations in conjunction with on-the-fly MP2 and CIS(D) electronic structure calculations. Whereas the former method relies on the classical approximation of nuclear motion, quantum effects on structure and dynamics are taken into account at finite temperatures by ab initio path integral and ab initio ring polymer molecular dynamics, respectively. It is shown that quantum-mechanical fluctuation effects of the nuclei, in addition to their purely thermal activation, cause significant configurational fluctuations due to strongly anharmonic vibrations and thus increase the explored regions on the Born-Oppenheimer potential energy surface at room temperature. Including these effects, in turn, leads to significant improvements in the computed spectra compared to stick spectra obtained at the equilibrium structure by means of harmonic normal-mode analysis, as well as by classical ab initio molecular dynamics. This family of methods, combining electronic structure with path integrals, offers a powerful and general computational approach not only to molecular structure determination of floppy or fluxional molecules, but also to evaluation of their electronic and vibrational spectra.