Accurate Quantum Dynamics of Rare-Gas Heteroclusters and Hydrogen-Bonded Dimers

本研究由美国国家科学基金会理论与计算化学项目资助。将进行严格的量子力学计算来研究弱束缚系统的内部动力学，特别是稀有气体异质团簇和氢键二聚体。该项目有三个主要组成部分。(1)将量子蒙特卡罗方法应用于由几个氩原子组成的卤化氢分子簇，研究溶质-溶质相互作用对能量学、结构和振动谱的影响。(2)将发展改进的计算方法来处理具有一个共价键的四原子团簇的旋转振动态。将明确处理所有五个弱键的大幅扭曲。(3)对特定模式的卤化氢二聚体的振动预离解动力学进行精确的量子计算，以获得预离解寿命和所得单体碎片旋转态分布的理论预测。在过去的十年中，分子束技术和激光光谱学的进步使得今天可以对气相中非常小的分子簇的振动和旋转状态的相对能量进行详细和高度精确的测量。这些实验是关于弱原子相互作用的重要的、高度精确的和详细的信息的潜在来源，但是需要进一步的理论工作来充分提取这些信息。目前的项目是对三种特定类型的团簇的理论研究，其大小从4到17个原子不等。这些团簇与具有相同原子数量的典型稳定分子的显著区别在于它们的极端软性。求解典型分子小振幅振动的量子力学运动方程的数学方法已经发展得很好。弱约束团簇中更复杂的大振幅运动理论是当前研究的一个活跃领域。除了固有的科学好奇心外，这项工作的动机是需要关于弱分子间力的定量信息，以便在随后的大分子系统中用于分子运动的计算机模拟，例如生物感兴趣的分子和液体溶液中的分子。

This research is supported by the NSF theoretical and computational chemistry program. Rigorous quantum mechanical calculations will be carried out to study the internal dynamics of weakly bound systems, particularly, rare gas heteroclusters and hydrogen-bonded dimers. The project has three main components. (1) Quantum Monte Carlo methods will be applied to clusters consisting of a hydrogen halide molecule with several argon atoms to study the effects of solvent-solute interaction on energetics, structure, and the vibrational spectrum. (2) Improved computational methods will be developed for treating rovibrational states of four-atom clusters with one covalent bond. Large amplitude distortions of all five weak bonds will be treated explicitly. (3) Accurate quantum calculations will be carried out for mode-specific vibrational predissociation dynamics of hydrogen halide dimers to obtain theoretical predictions of predissociation lifetimes and rotational state distributions of the resulting monomer fragments. Advances during the past decade in molecular beam technology and laser spectroscopy have made it possible today to carry out detailed and highly precise measurements of the relative energies of vibrational and rotational states of very small molecular clusters in the gas phase. These experiments are potential sources of important, highly accurate, and detailed information about weak interatomic forces, but further theoretical work is required to fully extract this information. The present project is a theoretical study of three particular classes of clusters ranging in size from 4 to 17 atoms. The significant difference between these clusters and typical stable molecules with the same number of atoms are their extreme floppiness. Well developed mathematicals methods already exist for solving the quantum mechanical equations of motion for the small amplitude vibrations of typical molecules. The theory of the more compli cated, large amplitude, motion in weakly bound clusters is an active area of current research. This work is motivated, aside from inherent scientific curiosity, by the need for quantitative information about weak intermolecular forces to be used in subsequent computer simulations of molecular motion in large molecular systems, e.g. molecules of biological interest and molecules in liquid solution.