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锥形交叉点周围的玻恩-奥本海默近似的崩溃是至关重要的，以便建模和改善光到能量的转换过程。这样的过程在光收集（例如天然和人工光合作用）中是基本的，并且还可以在具有对光的机械响应的分子尺度装置（天然和人工构建体中的光异构化和光感受器）中发挥作用。虽然原子核是重粒子，通常可以用经典的方法来处理，但光动力学的情况是一个显著的例外。光的吸收总是产生一个相干的初始波包，它在比退相干时间更快的时间尺度上保持其量子性质。因此，在几皮秒以下的时间尺度上的超快光化学本质上是量子过程。在激发态化学的建模中存在两个主要挑战。首先，势能面（PES）和它们的非绝热（非玻恩-奥本海默）耦合很难描述，因为它们需要激发电子态的电子结构。在大系统中，PES的拓扑结构可能非常复杂。第二，动力学的量子性质要求解多维波包的薛定谔方程。当轨道引导基组被用来描述量子动力学时，PES和波函数复杂性的问题都得到了解决。首先，从头计算必须仅沿沿着一组轨迹进行，其次，基组大小被最小化，因为轨迹引导基仅覆盖系统相空间的最重要部分。此外，具有随机选择的初始条件的轨迹引导基组避免了量子力学的所谓指数缩放与自由度的数量。在这个项目中，我们将进一步发展量子力学的两个前沿的轨道为基础的方法，即从头算多产卵（AIMS）和多构型Escherichfest（MCE）动力学。我们将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功能的方法。我们将应用新的方法来模拟化学和生物化学中的激发态动力学。最重要的是，我们还将开发新的相关方法。首先，我们将工作的半经典近似的新MCE方法，其次，我们将探索使用基于轨迹的方法来研究量子动力学的费米子，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.