Ab initio molecular dynamics simulations are presented to investigate the intramolecular vibrational energy redistribution (IVR) of an isolated nitromethane molecule. A number of IVR processes are simulated by monitoring the kinetic energy of vibrational modes under selective low-lying vibrational excitations from their ground states (Δν = 1 or 2). Evolution of the normal-mode kinetic energy gives the ultrafast energy transfer processes from parent modes to daughter modes intuitively. From the ultrafast vibrational transfer made by Fourier transformation of the time-dependent normal-mode kinetic energy, we can capture that the symmetry of the normal modes plays an important role in the anharmonic coupling between the vibrational modes. The results show three symmetry-dependent coupling mechanisms: direct symmetric coupling, overtone-assisted coupling, and rotation-assisted coupling. Furthermore, the calculated efficiencies of IVR also coincide with these mechanisms.

中文翻译：

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阐明硝基甲烷中快速分子内振动能量重新分布的耦合机理：从头算分子动力学模拟。

提出了从头算分子动力学模拟来研究分离的硝基甲烷分子的分子内振动能重新分布（IVR）。通过监视来自其基态（Δν= 1或2）的选择性低洼振动激发下的振动模式的动能，可以模拟许多IVR过程。正常模式动能的演变直观地实现了从父模式到子模式的超快速能量传输过程。从时变法向模态动能的傅立叶变换产生的超快振动传递中，我们可以捕获到，法向模态的对称性在振动模态之间的非谐耦合中起着重要作用。结果显示了三种依赖于对称性的耦合机制：直接对称耦合，泛音辅助耦合和旋转辅助耦合。此外，IVR的计算效率也与这些机制一致。