Coupling the Quantum Liouville Equation to Quasi-Classical Trajectories: Investigating EET in light harvesting molecular systems

自然光合作用是通过吸收太阳光的光子而引发的，该光子在自然光系统的光捕获叶绿素色素分子内产生激发电子态（即具有额外能量量子的电子）。这种激发的电子状态被称为“激子”，一旦它在光合生物的天线系统中产生，它就会在光合蛋白复合物中的许多叶绿素分子之间转移。激子最终到达光合反应中心（大约0.0000000001秒或1皮秒），在那里它的能量被用来引发化学反应，最终导致水分解和碳固定。正是这些反应最终为植物、藻类或细菌提供了维持它们所需的能量和食物。人工光合作用的新兴领域寻求设计人造分子装置来模仿自然光合系统。科学家和工程师们希望通过使用可以收集太阳能用于发电的设备来解决21世纪世纪世界能源问题，也许更重要的是，可再生生物燃料可以以与不可再生化石燃料相同的方式使用。人工光合作用领域是多学科的，涉及化学家，物理学家，工程师，生物学家和其他科学家和技术人员之间的合作。该项目旨在通过开发新型计算机模拟，对激子能量转移（EET）的基本过程（光合作用的初始过程）进行更深入的理论理解。EET是一个过程，其中来自太阳的光子被分子吸收，从而在分子内产生电子激发态。这种电子激发态，称为激子，然后转移到分子内的另一个位置，在那里多余的能量可以用来做功，通常以驱动化学反应的形式。虽然科学家对EET过程有了合理的理解，但在分子水平上仍有许多不了解的地方，这可能会影响捕光材料和人工光合装置的设计原理。最近的实验揭示了自然光系统中激子能量转移（EET）机制的基本问题。2007年，加州大学伯克利分校的研究人员用光谱法研究了一种天然光系统（FMO蛋白），并在生物系统中观察到长寿命的量子力学，即使在环境温度下也是如此。这是一个巨大的惊喜，因为人们认为生物系统太“大”和“温暖”，量子力学效应不可能长时间存在。观察到的影响是否在促进EET过程中很重要的问题是一个开放和有争议的问题之一。然而，了解这些量子效应在EET过程中是否重要，对于开发完全优化的人工光系统至关重要，这些人工光系统可用于在可用于发电和生产可再生燃料的实际设备中收集太阳能。该项目涉及混合量子/经典理论方法的发展，这将使我们能够在高性能并行计算机上模拟各种分子系统中的EET过程，类似于用于天气预报的方法。在这项工作中，我们将专注于模拟来自太阳的电磁辐射如何与激子相互作用，以及分子的运动如何反过来影响激子的（量子）演化。

Natural photosynthesis is initiated by the absorption of a photon of sunlight producing an excited electronic state (i.e. an electron with an additional quantum of energy) within the light harvesting chlorophyll pigment molecules of a natural photosystem. This excited electronic state is called 'the exciton' and once it is produced in the antenna system of a photosynthetic organism, it is transferred between numerous chlorophyll molecules within the photosynthetic protein complexes. The exciton eventually ends up in the photosynthetic reaction centre (within about 0.000000000001 of a second, or 1 picosecond), where its energy is used to initiate chemical reactions that eventually lead to water splitting and carbon fixation. It is these reactions that ultimately give the plants, algae or bacteria the energy and food required to sustain them.The emerging field of artificial photosynthesis seeks to engineer man made molecular devices that mimic natural photosynthetic systems. In doing this, scientists and engineers hope to address the world's energy concerns of the 21st century by employing devices that can harvest solar energy for the purpose of electricity and, perhaps more importantly, renewable biofuels that can be used in much the same way non-renewable fossil fuels are used. The field of artificial photosynthesis is multidisciplinary and involves collaborations between chemists, physicists, engineers, biologists, and other scientists and technologists. This project seeks to gain a deeper theoretical understanding of the fundamental process of exciton energy transfer (EET), the initial process of photosynthesis, through the development of new types of computer simulations. EET is the process in which photons, originating from the sun, are absorbed by a molecule giving rise to an electronic excited state within the molecule. This electronic excited state, called an exciton, is then transferred to another location within the molecule where this excess energy can be used to do work, generally in the form of driving a chemical reaction. Although scientists have a reasonable understanding of the EET process, there is still much that is not understood at the molecular level, which may have an impact the design principles of light harvesting materials and artificial photosynthetic devices. Recent experiments have opened up fundamental questions about the mechanism behind exciton energy transfer (EET) within the natural photosystems. In 2007 researchers at University of California, Berkeley, investigated one of the natural photosystems (the FMO protein) spectroscopically and observed long-lived quantum mechanical in biological systems, even at ambient temperatures. This was a great surprise because it was thought that biological systems were too 'large' and 'warm' for quantum mechanical effects to be present for long periods of time. The question of whether the observed effects are important in facilitating the EET processes is an open and controversial question one. Nevertheless, understanding whether these quantum effects are important in the EET process is vitally important in the race to develop fully optimised artificial photosystems that can be utilized for solar energy harvesting in practical devices that could be used to generate electricity and production of renewable fuels. This project involves the development of a mixed quantum/classical theoretical approach that will allow us to simulate EET processes in a variety of molecular systems on high performance parallel computers, similar to those used for weather forecasting. In this work, we will focus on simulating how electromagnetic radiation from the sun interacts with the exciton and how the movements of the molecule in turn affects the exciton's (quantum) evolution.