Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2012 Apr 7;136(13):134507. doi: 10.1063/1.3700718.
We present an approach for calculating nonlinear spectroscopic observables, which overcomes the approximations inherent to current phenomenological models without requiring the computational cost of performing molecular dynamics simulations. The trajectory mapping method uses the semi-classical approximation to linear and nonlinear response functions, and calculates spectra from trajectories of the system's transition frequencies and transition dipole moments. It rests on identifying dynamical variables important to the problem, treating the dynamics of these variables stochastically, and then generating correlated trajectories of spectroscopic quantities by mapping from the dynamical variables. This approach allows one to describe non-Gaussian dynamics, correlated dynamics between variables of the system, and nonlinear relationships between spectroscopic variables of the system and the bath such as non-Condon effects. We illustrate the approach by applying it to three examples that are often not adequately treated by existing analytical models--the non-Condon effect in the nonlinear infrared spectra of water, non-Gaussian dynamics inherent to strongly hydrogen bonded systems, and chemical exchange processes in barrier crossing reactions. The methods described are generally applicable to nonlinear spectroscopy throughout the optical, infrared and terahertz regions.
我们提出了一种计算非线性光谱观测值的方法，该方法克服了当前唯象模型中固有的近似，而不需要进行分子动力学模拟的计算成本。轨迹映射方法使用半经典近似线性和非线性响应函数，并从系统跃迁频率和跃迁偶极矩的轨迹计算光谱。它依赖于识别对问题重要的动力学变量，对这些变量的动力学进行随机处理，然后通过从动力学变量映射生成光谱量的相关轨迹。该方法允许描述非高斯动力学、系统变量之间的相关动力学以及系统与浴之间的非线性关系，例如非康顿效应。我们通过将其应用于三个示例来说明该方法，这些示例通常不能被现有分析模型充分处理——水的非线性红外光谱中的非康顿效应、强氢键系统固有的非高斯动力学以及势垒穿越反应中的化学交换过程。所描述的方法通常适用于光学、红外和太赫兹区域的整个非线性光谱。
