Quantum simulations of ultrafast photodynamics with the novel Multi-Configurational Ehrenfest technique

光吸收总是产生一个相干的初始量子波包，在低于退相干时间的时间尺度上保持其量子性质。因此，在亚皮秒时间尺度上的超快光化学，跟随光吸收，本质上是一个量子过程。光化学过程的理论研究是一项艰巨的任务。分子内部的相互作用可以是相当复杂的，分子的许多振动模式可以参与动力学。最大的挑战来自于这样一个事实，即在具有多自由度的复杂系统中，量子波包动力学用现有的计算方法进行数值模拟是非常昂贵的。计算成本随着自由度的增加呈指数增长，这通常被称为量子力学的指数诅咒。近年来，新的多构型Ehrenfest方法得到了发展，它明显地克服了指数诅咒，可以处理非常大系统中的量子动力学。该方法在具有数千个自由度的模型基准系统中准确地描述了量子动力学。现在的建议是将该方法与现有的电子结构代码连接起来，这些代码可以产生分子中原子之间的实际相互作用，而不是简单的模型，并从模型系统逐步转变为现实的模拟。有了这个新的从头算量子直接动力学，我们将研究一些以前在实验中研究过的日益复杂的光化学过程。我们将尝试了解吸收光的能量如何在参与光收集的分子中演变。

Light absorption always creates a coherent initial quantum wave packet which at the time scale lower than decoherence time retains its quantum nature. Therefore ultrafast photochemistry on the subpicosecond timescale, which follows light absorption, is an essentially quantum process. Theoretical study of photochemical processes is a difficult task. The interactions within the molecule can be quite complex and many vibrational modes of a molecule can be involved in the dynamics. The greatest challenge comes from the fact that quantum wave packet dynamics in complex systems with many degrees of freedom (DOF) is prohibitively expensive to simulate numerically with existing computational methods. The computational cost grows exponentially with the number of degrees of freedom which is often called the exponential curse of quantum mechanics. Recently the new Multi-Configurational Ehrenfest approach has been developed which apparently overcomes the exponential curse and can treat quantum dynamics in very large systems. The method outperformed the competing techniques and described accurately quantum dynamics in model benchmark systems with thousands of degrees of freedom. The proposal now is to connect the method with existing electronic structure codes which produce realistic interaction between atoms in molecules as opposed to simple models and to make a step change from model systems to realistic simulations.With this new ab initio quantum direst dynamics we will study a number of photochemical processes of increasing complexity previously investigated experimentally. We will try to understand how the energy of absorbed light evolves in the molecules involved in light harvesting.