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

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.

The innovation of this project lies in the development and application of a new method of quantum dynamics, Multi-Configurational Ehrenfest (MCE). Only a handful of techniques capable of treating quantum systems with many degrees of freedom exist and as it is argued in the case for support the MCE appears to be uniquely suited for developing ab initio quantum direct dynamics. The project will apply MCE to the simulation of ultrafast photodynamics and in particular to that of light harvesting where subpicosecond evolution is essentially quantum even in large and heavy molecules. Classical ab initio direct dynamics methods are becoming a useful and almost routine tool for chemists. They treat electronic structure with quantum ab initio methods and nuclear motion classically, relying on surface hopping to describe nonadiabatic dynamics. Ab initio direct dynamics allows predictions of rates, which paves the way to intelligent design and synthesis of complex molecules with desired functions. If successful the current project will provide a working simulation technique, which treats nuclear degrees of freedom also quantum mechanically with the MCE method. Many current experiments reveal quantum features of the nuclear dynamics and only can be modelled by a fully quantum approach. It is widely accepted that coherent quantum interference effects play an important role in the dynamics of small molecules. There is growing experimental evidence that such effects are important in large biologically related species as well. As it is shown in the case for support the existing method of Multiple Spawning, which in principle includes quantum equations, effectively operates as a classical surface hopping technique. The proposed method will represent a significant improvement and will be able to describe quantum effects in photodynamics of large molecules. A fully quantum ab initio direct dynamics technique will find many other applications. For example simulations of charge and energy transfer in systems related to molecular electronics, which currently are done with the Multiconfigurational Time-Dependent Hartree method (MCTDH) using model potential energy surfaces, will be done with an ab initio technique. MCE can also be used for simulations of chemical dynamics in big molecules and in condensed phase and for dynamics on surfaces just like any classical ab initio direct dynamics method The project is a part of a broader effort to provide new methods capable of treating a large number of quantum degrees of freedom. The previously developed Coupled Coherent States (CCS) method, which is related to MCE, has been applied before to many processes, ranging from vibrational energy exchange in polyatomic molecules to dynamics of electrons in a strong laser field. Currently we are working with quantum computing group of Dr. Alessio Serafini at UCL (London) to use MCE for simulations of decoherence in quantum computers. Another potential area of applications is Coherent Control. Many more areas could be thought of as well. Producing computational methods, which avoid the exponential curse of quantum mechanics and therefore can treat many-body quantum systems is one of the central problems of computational computational physics and chemistry. The technologies of the future will rely more and more on the quantum properties of matter and their development will require fully quantum computational techniques. On a more fundamental level new methods like MCE improve our understanding of the mathematical structure of quantum mechanics. Also needless to say that developing such techniques is a huge intellectual challenge. Therefore the code and the project publications will have very strong impact on a broader scientific community.