RUI: Correlated Methods for Calculation of the Electronic Coupling Element

Harvey Mudd学院的Robert Cave获得了化学系化学理论、模型和计算方法项目的奖励，他开发了研究电子转移反应的计算方法。电子转移反应是最简单的化学反应，但在生物系统和寻求利用太阳能作为替代能源的人类工程装置中起着核心作用。植物光合作用的初始阶段是经过精心设计的，可以在远距离转移电子，尽可能少地浪费能量，以便产生高能分子供以后使用。动物细胞中的线粒体使用类似的“反向”电子转移途径来合成分子，使高能前体的肌肉运动。当我们试图模仿植物利用太阳能的有效性时，在详细水平上理解电子转移过程是至关重要的。电子转移速率取决于许多量，但控制速率的距离和方向依赖的因素被称为“电子耦合元素”，如果我们能在基本水平上理解它，这个量有可能产生前所未有的速率控制。凯夫和他的同事开发了计算电子耦合的新方法，比现有的方法更精确。这些新方法和结果将用于更好地设计新的合成太阳能转换装置和了解生物电子转移机制。这项工作是由本科院校哈维马德学院进行的。这项工作为可能攻读化学研究生的学生提供了一个极好的教育机会，同时也支持了向女性工程师协会（Society of Women Engineers）举办的理论化学研讨会的年度报告。高中女生校园日，100-150名高中女生被介绍到科学、工程和数学领域。在过去的二十年中，从标准量子化学方法中提取电子耦合的硅化技术取得了巨大的进步。然而，由于相关量子化学方法与系统尺寸的标度差，人们经常面临精度和可追溯性之间的选择，特别是对于大型电子转移系统。鉴于这些数字化方法的成功，必须将注意力转向开发新的电子结构理论方法，为电子转移问题量身定制，这些方法是准确的，能够处理相当大的系统。这项工作通过开发一系列近似方法来解决这一挑战，这些方法包括与电子转移过程相关的所有零阶态以平衡的方式相关。这些方法极大地扩展了电子传递系统的尺寸，从而可以准确地获得耦合，从而为更近似方法的适用性提供了有用的指导。特别地，基于运动耦合簇方法的近似，开发了一组相关的方法来计算电子耦合元件。这些方法的可伸缩性并不比MP2/MBPT2差，可以访问比CI或CCSD大得多的系统。使用基于pt的系数代替相似变换哈密顿量中的CCSD系数，再加上为电子转移系统定制的截断激励空间，导致范围和速度的增加。这些新方法的测试包括一系列可以进行高精度计算的模型系统，应用于计算近简并给体/受体和桥系统中的电子耦合，以及通过溶剂的电子转移的研究。对后两种计算目标均进行了实验研究。

Robert Cave of Harvey Mudd College is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop computational methods for studying electron transfer reactions. Electron transfer reactions are the simplest chemical reactions but have central roles in biological systems and in human-engineered devices that seek to harness solar energy as alternative sources. The initial stages of plant photosynthesis are exquisitely engineered to transfer electrons across large distances, wasting as little energy as possible, in order to make high-energy molecules for later use. The mitochondria in the cells of animals use an analogous "reverse" electron transfer pathway to synthesize molecules that allow muscle movement from high-energy precursors. As we seek to mimic the effectiveness of plants in harnessing solar energy it is critical to understand electron transfer processes at a detailed level. The rate of electron transfer depends on many quantities, but the factor that controls the distance- and orientation-dependence of the rate is called the "electronic coupling element", a quantity that has the potential to yield unprecedented control of rates if we can understand it at a fundamental level. Cave and his coworkers develop new methods for the calculation of the electronic coupling that are more accurate than existing approaches. The new methods and results are used to better design new synthetic solar energy conversion devices and understand mechanisms of biological electron transfer. The work is carried out Harvey Mudd College, an undergraduate institution. The work provides an excellent educational opportunity for students likely to pursue graduate work in chemistry and also supports the annual presentation of a theoretical chemistry workshop to the Society of Women Engineers? On-Campus Day for high school women, where 100-150 high school women are introduced to opportunities in science, engineering and mathematics.Great strides have been made over the past two decades in developing diabatization techniques for extracting the electronic coupling from standard quantum chemical approaches. However, because of the poor scaling of correlated quantum chemistry methods with system size one is often faced with the choice between accuracy and tractability, especially for large electron transfer systems. Given the success of these diabatization methods, it is imperative to turn attention to developing new electronic structure theory approaches, tailored to the electron transfer problem, which are accurate and able to treat considerably larger systems. This work addresses this challenge by developing a series of approximate methods that include correlation in a balanced fashion for all of the zeroth-order states relevant to the electron transfer process. These approaches greatly extend the size of electron transfer systems for which the coupling can be obtained accurately and thus provide useful guidance about the suitability of more approximate methods. In particular a family of correlated methods is developed to calculate the electronic coupling element based on approximations to the Equation of Motion Coupled Cluster approach. The methods scale no worse than MP2/MBPT2, giving access to dramatically larger systems than available to CI or CCSD. Use of PT-based coefficients in place of CCSD coefficients in the similarity transformed Hamiltonian, coupled with truncated excitation spaces tailored to electron transfer systems, lead to the increased scope and speed. Tests of these new methods include a series of model systems where high-accuracy calculations can also be performed, application to calculate the electronic coupling in near-degenerate donor/acceptor and bridge systems, and the study of through-solvent electron transfer. Both of the latter two computational targets have been studied experimentally.