Conical Intersections

Abstract

INTRODUCTIONMany of us who are involved in teaching a special-topic graduate course may have the experience that it is difficult to find suitable references, especially reference materials put together in a suitable text format. Presently, several excellent book series exist and they have served the scientific community well in reviewing new developments in physical chemistry and chemical physics. However, these existing series publish mostly monographs consisting of review chapters of unrelated subjects. The modern development of theoretical and experimental research has become highly specialized. Even in a small subfield, experimental or theoretical, few reviewers are capable of giving an in-depth review with good balance in various new developments. A thorough and more useful review should consist of chapters written by specialists covering all aspects of the field. This book series is established with these needs in mind. That is, the goal of this series is to publish selected graduate texts and stand-alone review monographs with specific themes, focusing on modern topics and new developments in experimental and theoretical physical chemistry. In review chapters, the authors are encouraged to provide a section on future developments and needs. We hope that the texts and review monographs of this series will be more useful to new researchers about to enter the field. In order to serve a wider graduate student body, the publisher is committed to making available the monographs of the series in a paperbound version as well as the normal hardcover copy. Cheuk-Yiu Ng PREFACEThe Born-Oppenheimer adiabatic approximation represents one of the cornerstones of molecular physics and chemistry. The concept of adiabatic potential-energy surfaces, defined by the Born-Oppenheimer approximation, is fundamental to our thinking about molecular spectroscopy and chemical reaction dynamics. Many chemical processes can be rationalized in terms of the dynamics of the atomic nuclei on a single BornOppenheimer potential-energy surface. Nonadiabatic processes, that is, chemical processes which involve nuclear dynamics on at least two coupled potential-energy surfaces and thus cannot be rationalized within the BornOppenheimer approximation, are nevertheless ubiquitous in chemistry, most notably in photochemistry and photobiology. Typical phenomena associated with a violation of the Born-Oppenheimer approximation are the radiationless relaxation of excited electronic states, photoinduced unimolecular decay and isomerization processes of polyatomic molecules. During the last few decades, we have witnessed a change of paradigms in nonadiabatic chemistry. First, the remarkable advances achieved in femtosecond laser technology and time-resolved spectroscopy have revealed that the radiationless decay of excited electronic states may take place much faster than previously thought. The traditional theory of radiationless decay processes, developed in the sixties and seventies of the last century, cannot explain electronic decay occurring on ...

Analytical Derivative Coupling for Multistate CASPT2 Theory

“…(19) is the scalar coupling. 7,8,22,25,26 The derivative coupling formulas for (X)MS-CASPT2 can be obtained in a similar fashion. The Lagrangians for the zeroth-order energy and the energy corrected to second order (the Hylleraas functional) are…”

“…Efficient computation of derivative couplings together with nuclear energy gradients enables on-the-fly dynamics simulations of photochemical processes. 2,[22][23][24] It also allows for locating conical intersections between potential energy surfaces, 4,[25][26][27][28][29] which are the set of geometries where two or more potential energy surfaces intersect with each other. Since the computational costs of these applications are strongly dominated by the underlying computation of the derivative couplings and nuclear energy gradients, development of quantum chemical approaches for their efficient and accurate evaluation has the potential to significantly advance what is considered to be the state of the art method in computational photophysics and photochemistry.…”

Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

“…59 Nonradiative electronic decay on the femtosecond time scale is now widely reported for a large variety of molecular systems 60À64 and believed to be frequently mediated by conical intersections. 65,66 Whether a conical intersection is also responsible for the observed ultrafast nonradiative decay in P3HT films has to be clarified in the future. It has recently been established in conjugated polymers that exciton localization occurs in tens of femtoseconds and leads to nonemissive states.…”

Three-State Conical Intersections in Nucleic Acid Bases

“…4 Because most molecular systems in nature have low or no symmetry, these accidental intersections may have a great impact on the photophysics and photochemistry of molecular systems, as has been found in accidental two-state intersections. 1,2,5,6 Three-state degeneracies can provide a more efficient relaxation pathway when more than one interstate transition is needed, they introduce more complicated geometric phase effects, [7][8][9] and they can affect the dynamics and available pathways. 10 The exact way these features affect the dynamical behavior of molecules is a question that has yet to be studied extensively.…”