Conic intersections overview

In order to be able to characterize the PES of excited states, locate conical intersections, and derive mechanisms for photophysics and photochemistry, efficient electronic structure methods for excited states are required. In the following section we give a brief overview of the current state of methodological developments in electronic structure methods applicable to excited states. 

Most of the AIMD simulations described in the literature have assumed that Newtonian dynamics was sufficient for the nuclei. While this is often justified, there are important cases where the quantum mechanical nature of the nuclei is crucial for even a qualitative understanding. For example, tunneling is intrinsically quantum mechanical and can be important in chemistry involving proton transfer. A second area where nuclei must be described quantum mechanically is when the BOA breaks down, as is always the case when multiple coupled electronic states participate in chemistry. In particular, photochemical processes are often dominated by conical intersections [14,15], where two electronic states are exactly degenerate and the BOA fails. In this chapter, we discuss our recent development of the ab initio multiple spawning (AIMS) method which solves the elecronic and nuclear Schrodinger equations simultaneously this makes AIMD approaches applicable for problems where quantum mechanical effects of both electrons and nuclei are important. We present an overview of what has been achieved, and make a special effort to point out areas where further improvements can be made. Theoretical aspects of the AIMS method are... 

In this article we have given an overview over our theoretical studies on tlie multi-mode multi-state vibronic interactions in the benzene cation and several of its fluoro derivatives. These are all associated with multiple conical intersections between the... 

In the previous section we gave an overview of some general reaction profiles for photochemistry and highlighted the role of surface crossings. We will now give a description of surface crossings as conical intersections in simple theoretical... 

In this section we examine conical intersections, the derivative coupling, the geometric phase, and how they affect nonadiabatic processes. We therefore begin with an overview of these concepts and the Born-Huang approach from which they originate. 

As well as providing an overview of the MCTDH method and the vibronic coupling Hamiltonian, results will be presented in this chapter that highlight the features of three photo-physical systems in which vibronic coupling plays a crucial role. Here it will become clear that in some cases many degrees of freedom are required for a faithful description of the system dynamics through a conical intersection, and for these the MCTDH method is able to provide accurate information. 

In this chapter we briefly review both the pertmbative as well as nonperturbative formalism for the calculation of femtosecond time-resolved spectra, focusing on the detection of the photophysical and photochemical dynamics at conical intersections. To set the stage for a general discussion of the potential of modern time-resolved spectroscopy, we begin with a brief overview of the spectroscopic techniques, which are particularly suitable in the present context. 

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