Mixed Quantum/Semiclassical studies of spectroscopy and dynamics

在这项由化学系化学理论、模型和计算方法计划资助的项目中，俄勒冈大学的杰弗里·A·西纳教授正在开发理论方法来模拟和解释基于吸收光的低温材料的依赖于时间的激光测量。这种基于分子的方法近似于量子力学行为，使人们有可能进行高效的计算，尽管这些系统很复杂。这种方法还允许根据参与的分子和原子的运动来解释这些过程，很容易可视化。正在开发的理论框架称为固定振动基础/高斯浴描述。为了模拟光谱信号，它是以相对于几个外部激光脉冲的电场强度的微扰形式开发的。该理论通过物理激励的一组依赖时间的参数来描述多体化学系统的微观动力学，与直接基组展开所需的线性系数集合相比，这些参数的数量较少。最初的应用是嵌入稀有气体主体晶体中的共振驱动小分子靶。这类系统显示出许多凝聚相化学动力学的一般特征，同时由于低温下量子力学效应的突出而具有挑战性，并且易于接受量子/半经典混合处理。计算信号的质量正在通过与大量的实验数据和更密集的计算理论结果进行比较来评估，包括多组态依赖于时间的Hartree计算。

In this project funded by the Chemical Theory, Models and Computational Methods program of the Chemistry Division, Professor Jeffrey A. Cina of the University of Oregon is developing theoretical methods for simulating and interpreting time-dependent laser-based measurements of low-temperature materials that absorb light. The molecular-based approach, that approximates the quantum mechanical behavior, makes it possible to carry out efficient calculations despite the complexity of these systems. This method also allows the interpretation of these processes in terms of motion, easily visualized, of the participating molecules and atoms.The theoretical framework under development is termed a fixed vibrational basis/Gaussian bath description. For the purpose of simulating spectroscopy signals, it is being developed in perturbative form with respect to the electric field strengths of several external laser pulses. The theory describes the microscopic dynamics of many-body chemical systems in terms of a physically motivated set of time-dependent parameters that are small in number compared with the collection of linear coefficients that are required in a direct basis-set expansion. Initial applications are being made to resonantly driven small-molecule targets embedded in rare-gas host crystals. Systems of this kind display many generic features of condensed-phase chemical dynamics, while being both challenging, due to the prominence of quantum mechanical effects at cryogenic temperatures, and amenable to a mixed quantum/semi-classical treatment. The quality of the calculated signals is being assessed by comparison with extensive experimental data and more computationally intensive theoretical results, including multi-configuration time-dependent Hartree calculations.