Toward a deeper understanding of the quantum nature of molecular collisions

马里兰大学帕克分校的米勒德·亚历山大（Millard Alexander）获得了化学学部化学理论、模型和计算项目的奖励，他基于量子力学对化学碰撞（包括化学反应）进行了严格的理论研究。该奖项由化学部门的化学结构、动力学和机制- a （CSDM-A）项目和物理部门的理论原子、分子和光学物理（TAMOP）项目共同资助。化学的本质是原子从一种排列到另一种排列的重新排列。化学反应类似于驾车从一个山谷穿过山口进入另一个山谷。这种状态被称为“过渡状态”。“反作用力”发生的速度取决于分隔山谷的山口的高度、进入山口的难易程度以及山口被翻越的速度。实验仅限于前后对比，监测有多少汽车从最初的谷中消失，有多少汽车到达新的谷中，以及反应后原子的速度。由此，化学家试图推断化学键如何影响山口的形状，从而推断化学变化发生的速度。亚历山大开发的理论模型提供了补充实验的洞察力。亚历山大的研究重点是量子力学的不确定性如何影响化学反应性。该研究涉及天体化学、大气化学、超冷化学等多个领域。亚历山大小组开发和传播供研究界使用的计算机软件。在经典模拟的基础上，可以很好地理解小分子碰撞动力学的许多方面。尽管如此，量子处理为许多基本反应提供了唯一完全正确的描述，特别是那些涉及氢原子的反应；在完全解决的州与州之间；低温下；或者当两个（或更多）电子势能面同时被访问时。在后一种情况下，与范式双缝情况完全一样，只有量子力学才能描述进入不同势能表面的轨迹之间的干涉。亚历山大小组的研究目标是找出量子效应——共振，或者波恩-奥本海默近似中的分解——在分子动力学的广阔经典领域中表现出来。复杂的量子化学技术被用来为范式反应开发精确的势能面。然后将这些用于量子散射截面的计算，并最终计算速率常数。亚历山大的工作基本上是基于内部开发的计算机代码，继续为世界各地的一些顶级实验小组提供洞察力和理解。

Millard Alexander of the University of Maryland, College Park, is supported by an award from the Chemical Theory, Models and Computational program in the Chemistry division to conduct rigorous theoretical studies, based on quantum mechanics, of chemical collisions, including chemical reactions. The award is cofunded by the Chemical Structures Dynamics and Mechanisms-A (CSDM-A) program, also in the Chemistry Division, and the Theoretical Atomic, Molecular and Optical Physics (TAMOP) program in the Physics division. The essence of chemistry is the rearrangement of atoms from one arrangement to another. A chemical reaction is akin to driving from one valley over a mountain pass into another valley. The pass is called a "transition state". The rate at which "reaction" occurs is controlled by the height of the pass which separates the valleys, the ease of access to the pass, and the speed at which the pass is surmounted. Experiments are limited to a before and after view, monitoring how many automobiles disappear from the initial valley, how many arrive in the new valley, and the velocity of the atoms after reaction. From this, the chemist tries to infer how chemical bonding affects the shape of the mountain pass and, thus, the rate at which chemical change occurs. Theoretical models, of the kind that Alexander develops, provide insight which complements experiment. Alexander's work is particularly focused on how the uncertainty of quantum mechanics influences chemical reactivity. The research has relevance for many fields including astrochemistry, atmospheric chemistry and ultra-cold chemistry. The Alexander group develops and disseminates computer software for use by the research community. Many aspects of the dynamics of small-molecule collisions can be well understood on the basis of classical simulations. Notwithstanding, a quantum treatment provides the only fully correct description of many elementary reactions, especially those involving H atoms; at the fully-resolved state-to-state level; at low temperature; or when the two (or more) electronic potential energy surfaces are accessed simultaneously. In the latter case, exactly as in the paradigm two-slit situation, only quantum mechanics can describe the interference between trajectories which access the distinct potential energy surfaces. To piece out where quantum effects -- resonances, or the breakdown in the Born-Oppenheimer approximation -- show themselves within the vast classical domain of molecular dynamics is the goal of research in the Alexander group. Sophisticated quantum-chemical techniques are used to develop accurate potential energy surfaces for paradigm reactions. These are then used in quantum scattering calculations of cross sections and, ultimately, rate constants. Alexander's work, based substantially on computer codes developed in house, continues to provide insight and understanding for a number of top experimental groups worldwide.