Beyond Born–Oppenheimer: Non-adiabatic effects in accurate nuclear motion calculations of polyatomic molecules

Project ID: 2531ad1571

(You will need this ID for your application)

UCL Lead department: Physics and Astronomy

Project Summary:

Nonadiabatic effects are crucial in many areas of physics and chemistry, especially where the Born–Oppenheimer approximation - the separation of electronic and nuclear motions - fails. Although widely applicable, this approximation breaks down in important cases, such as in the photochemistry of polyatomic molecules, where multiple electronic states are energetically close and nuclear motions are complex. This breakdown often manifests in phenomena like conical intersections, which provide efficient pathways for radiationless decay between electronic states and can be detected spectroscopically. Such intersections reveal intricate coupling between electrons and nuclei, demanding approaches beyond Born–Oppenheimer. One notable manifestation of nonadiabaticity is the Berry phase, a topological phase arising under certain conditions of the adiabatic treatment of nuclear motion. The Berry phase adds depth to molecular theory by accounting for conical intersections’ impact on nuclear and electronic motion. Properly describing nonadiabatic effects thus requires sophisticated methods that can capture the dynamic interplay between coupled potential energy surfaces and provide detailed representations of electronic structure, excited states, and nonadiabatic couplings.

The aim of the project is to develop a full non-adiabatic, symmetry adapted, rotation-vibrational electronic computational method from first principles for polyatomic systems characterized by conical intersections. By advancing beyond current quasi-static methods, the proposed approach will improve the quantum mechanical treatment of nuclear motion in systems with conical intersections, meeting the demands of high-resolution spectroscopy. The method will retain the many advantages of the current Born-Oppenheimer procedures already developed by the research group and use these to provide a starting point to model beyond the Born-Oppenheimer effects introduced by inclusion of non-adiabatic couplings.