RUI: Correlated Methods for Calculation of the Electronic Coupling Element

哈维·马德学院的罗伯特·凯夫得到了化学系化学理论、模型和计算方法项目的支持，以开发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设备中发挥着核心作用。植物光合作用的最初阶段经过精心设计，以远距离传输电子，尽可能少地浪费能量，以制造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允许肌肉运动的高能前体分子。当我们试图模仿植物利用太阳能的有效性时，在详细的水平上理解电子转移过程是至关重要的。电子转移的速度取决于许多量，但控制速度与距离和方位相关的因素被称为“电子耦合元素”，如果我们能从根本上理解它，这个量有可能产生前所未有的速度控制。凯文和他的同事开发了比现有方法更准确的计算电子耦合的新方法。新的方法和结果被用来更好地设计新的合成太阳能转换装置，并了解生物电子转移的机理。这项工作是由哈维马德学院进行的，这是一所本科院校。这项工作为可能从事化学研究生工作的学生提供了一个极好的教育机会，并支持每年向女工程师协会提交理论化学研讨会。高中女生校园日，向100-150名高中女生介绍科学、工程和数学方面的机会。在过去的20年里，在开发从标准量子化学方法中提取电子耦合的脱辉绿技术方面取得了长足的进步。然而，由于关联量子化学方法在系统规模上的缩放性较差，人们经常面临在精度和可操作性之间的选择，特别是对于大型电子转移系统。鉴于这些脱氢方法的成功，必须将注意力转向开发新的电子结构理论方法，这些方法是针对电子转移问题量身定做的，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，其中包括以平衡方式对与电子转移过程相关的所有零级态进行关联。这些方法极大地扩展了电子转移系统的规模，从而可以准确地获得耦合，从而为更接近的方法的适用性提供有用的指导。特别地，基于运动方程耦合簇法的近似，发展了一系列相关的方法来计算电子耦合元件。这些方法的可扩展性不逊于MP2/MBPT2，允许访问比CI或CCSD可用的大得多的系统。在相似变换的哈密顿量中使用基于PT的系数代替CCSD系数，再加上为电子转移系统量身定做的截断激发空间，导致了范围和速度的增加。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，应用于计算近简并施主/受主和桥系统中的电子耦合，以及通过溶剂进行电子转移的研究。对后两种计算目标都进行了实验研究。

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.