International Collaboration in Chemistry: New First Principles Methods for Nonadiabatic Dynamics

详细了解电子激发态的动力学和圆锥形交点周围玻恩-奥本海默近似的击穿对于模拟和改进光到能量的转换过程至关重要。这些过程是光收集的基础，如自然和人工光合作用，也可能在具有机械响应光的分子尺度设备中发挥作用（自然和人工结构中的光异构化和光感受器）。虽然原子核是重粒子，通常可以经典地处理，但光动力学的情况是一个显著的例外。光吸收总是产生一个相干的初始波包，它在时间尺度上比退相干时间更快地保持其量子性质。因此，时间尺度小于几皮秒的超快光化学本质上是一个量子过程。在激发态化学的建模中有两个主要的挑战。首先，势能面（PES）及其非绝热（非born - oppenheimer）耦合难以描述，因为它们需要激发态的电子结构。在大型系统中，PES的拓扑结构可能非常复杂。其次，动力学的量子性质要求在许多维度上解出波包的Schrödinger方程。利用轨迹引导基集描述量子动力学时，既解决了粒子动力学问题，又解决了波函数复杂性问题。首先，从头计算只能沿着一组轨迹进行，其次，基集的大小被最小化，因为轨迹制导基只覆盖了系统相空间中最重要的部分。随机选择初始条件的轨迹引导基集也避免了量子力学中所谓的自由度指数标度问题。在本项目中，我们将进一步发展两种前沿的基于轨迹的量子力学方法，即从头算多重生成（AIMS）和多构型埃伦费斯特（MCE）动力学。我们将把MCE添加到现有的AIMS代码中，比较这两种方法，并开发一种结合它们最佳特性的方法。我们将把这些新方法应用于化学和生物化学中激发态动力学的模拟。最重要的是，我们还将开发新的相关方法。首先，我们将对新的MCE方法进行半经典近似，其次，我们将探索使用基于轨迹的方法来研究费米子的量子动力学，这将是一个全新的方向。

A detailed understanding of the dynamics of electronic excited states and the breakdown of the Born-Oppenheimer approximation around conical intersections is critical in order to model and improve light to energy conversion processes. Such processes are fundamental in light harvesting such as natural and artificial photosynthesis and also may play a role in molecular scale devices with mechanical responses to light (photoisomerization and photoreceptors in both natural and artificial constructs). Although nuclei are heavy particles and usually can be treated classically, the case of photodynamics is a significant exception. Light absorption always creates a coherent initial wave packet which retains its quantum nature on time scales faster than the decoherence time. Therefore ultrafast photochemistry on the timescale below few picoseconds is an essentially quantum process. There are two main challenges in modelling of excited state chemistry. First, the potential energy surfaces (PES) and their nonadiabatic (non-Born-Oppenheimer) couplings are difficult to describe because they require the electronic structure of excited electronic states. In big systems the topology of the PES can be very complicated. Second, the quantum nature of the dynamics requires the solution of the Schrödinger equation for the wave packet in many dimensions. Both problems of PES and wave function complexity are addressed when trajectory guided basis sets are used to describe quantum dynamics. First, ab initio calculations have to be done only along a set of trajectories and, second, the basis set size is minimized because trajectory guided basis covers only the most important part of the system's phase space. Also trajectory guided basis sets with randomly selected initial conditions avoid the so called exponential scaling of quantum mechanics with the number of degrees of freedom. In this project we will develop further two cutting edge trajectory based methods of quantum mechanics, namely ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) dynamics. We will add MCE to the existing AIMS code, compare the two methods, and develop an approach which combines their best features. We will apply the new methods to simulations of excited state dynamics in chemistry and biochemistry. Most importantly we will also develop new related methods. First we will work out semiclassical approximations to the new MCE method and second we will explore the use of trajectory based methods to study the quantum dynamics of Fermions, which would be a completely new direction.