Quantum Mechanical Studies of Interacting Modes in Molecular Systems

这个为期三年的理论研究项目由美国国家科学基金会理论与计算化学计划和材料理论计划共同资助。理论模型将用于研究：(1)玻璃的低温热学和光学性质；(2)分子聚集体的光学和电子性质以及电子和振动状态。主题(1)涉及使用经验力场模型进行计算机模拟以生成大样本的玻璃结构。每个生成的结构在势能面上搜索具有相关双阱势（DWP）的邻近局部最小值对。隧道对低温比热、导热系数和声子-发色团相互作用的贡献将使用DWP参数的统计分布来计算。主题(2)是分子聚集体、链和多原子分子中电子-振动（声子）耦合的量子和统计力学分析。将理论与低温玻璃的光学减相实验结果进行比较。这项研究的总体目标是了解并最终预测玻璃和其他无序材料在原子结构方面的光学和热性质。在原子水平上，玻璃缺乏完美单晶的规则周期性。这种无序性要求人们适当地考虑相关结构特征的统计分布。特别地，存在着能量几乎相同的结构对，它们可以通过几个原子的协调运动，加上周围许多原子的微小调整，把一个结构转换成另一个结构。双阱势是玻璃在这一转换过程中势能变化的数学表示。对于某些结构对来说，能量势垒非常小，对于这些结构，即使玻璃冷却到液氦温度，其他原子运动已经冻结，相互转换运动仍然存在。因此，低温实验允许人们研究隧道运动的影响，而不受其他类型运动的影响。然后将根据计算得到的分布计算出玻璃的各种可测量的光学和热性能，以检验该理论。

This three-year theoretical research project is jointly supported by the NSF Theoretical and Computational Chemistry Program and the Materials Theory Program. Theoretical models will be studied for the study of: (1) low temperature thermal and optical properties of glasses, and (2) optical and electronic properties and electronic and vibrational states of molecular aggregates. Topic (1) involves computer simulations to generate a large sample of glass structures using an empirical force field model. Each generated structure is searched for pairs of neighboring local minima on the potential energy surface with associated double well potentials (DWP). Tunneling contributions to low temperature specific heat, thermal conductivity and phonon-chromophore interactions will be computed using the statistical distribution of DWP parameters. Topic (2) is a quantum and statistical mechanical analysis of electronic-vibrational (phonon) coupling in molecular aggregates, chains, and polyatomic molecules. Theory will be compared with results of optical dephasing experiments on low temperature glasses. The overall goal of this research is to understand, and eventually to predict, optical and thermal properties of glasses and other disordered materials in terms of their atomic structure. At the atomic level, a glass lacks the regular periodicity of a perfect single crystal. This disorder requires that one take proper account of the statistical distribution of relevant structural features. In particular, there exist pairs of structures of nearly the same energy which can be converted one into the other by a concerted motion of just a few atoms with small adjustments by the many surrounding atoms. The double well potential is a mathematical representation of the change of the potential energy of the glass during this conversion process. The energy barrier is very small for some pairs of structures, and for these, interconversion motions persist ev en when the glass is cooled to liquid helium temperatures, where other atomic motions have been frozen out. Thus low temperature experiments allow one to study the effects of the tunneling motion free from complications due to other types of motion. Various measurable optical and thermal properties of the glass will then be computed from the computed distribution to test the theory.