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

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

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.

Planned Impact

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.

Publications

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Kirrander A (2016) Ultrafast X-ray Scattering from Molecules. in Journal of chemical theory and computation

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Booth J (2014) Recent applications of boxed molecular dynamics: a simple multiscale technique for atomistic simulations. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

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Saita K (2013) Simulation of ultrafast photodynamics of pyrrole with a multiconfigurational Ehrenfest method. in Physical chemistry chemical physics : PCCP

 
Description Within this project we performed atomistic simulations in which dynamics of ultrafast chemical reactions was treated on a fully quantum level. All electrons and all nuclei were simulated solely on the basis of the equations of quantum mechanics. The quality of the result was good enough to reproduce and explain velocity map imaging experiments. This was the first demonstration that such calculation is possible.
Exploitation Route The theory is now used by the experimental group of Vas Stavros from Warwick and by us to simulate more experiments and to unravel the mechanisms of ultrafast chemical reactions of a number o molecules which represent the building blocks of larder biomolecules. The aim of these experiments is to understand how light can damage biomolecules and what factors determine the photostability of biomolecules.
Sectors Chemicals,Energy,Other

 
Description We performed a number of quantum dynamics simulations, which shed light on the processes which occur in light harvesting. The simulations explain what happens in the first several hundreds of femtoseconds after a molecule absorbs light. The ideas and methods developed in this project have been used later by us and by other groups to simulate intramolecular energy transfer in conjugate molecules and understand how the structure of such molecules influences energy transport in them. We also used the methods developed in this project to simulate ultrafast photochemistry.
First Year Of Impact 2015
Sector Chemicals,Education,Energy,Other
 
Description Uderstanding the mechanisms of photostability of biochemical building blocks from quantum simulations and imaging experiments.
Amount £298,680 (GBP)
Funding ID RPG-215-190 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 01/2015 
End 12/2018
 
Description CMS 2012 Cirencester, Direct Dynamics for Nonadiabatic Systems: the New Ab Initio Multiconfigurational Ehrenfest Method 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact A poster by Kenichiro Saita, Dmitrii V. Shalashilin, "Direct Dynamics for Nonadiabatic Systems: the New Ab Initio Multiconfigurational Ehrenfest Method" was presented at the conference Computational Molecular Science 2012 (CMS2012), 24-27 June 2012, Poster 23, Royal Agricultural College (Cirencester, United Kingdom).

The work was presented and discussed with colleagues and experts in the field
Year(s) Of Engagement Activity 2012
URL http://www.chm.bris.ac.uk/cms/