A more complete picture of photochemistry gives us the ability to predict reaction products and trends in photochemical reactions, with potential applications in light harvesting, molecular machines and organic synthesis.
Photochemical reaction pathways and conical intersection seams
Molecular dynamics are governed by potential energy surfaces, which describe the energies of electronic states of a molecule for all changes in the positions of atomic nuclei. Two electronic states can meet at a conical intersection seam, which acts like a funnel that takes a molecular wavefunction between electronic states without emitting a photon.
Much of photochemistry involves downhill reactions from an excited electronic state back to the ground state, meaning the molecular wavefunction has enough initial energy to access many regions of the intersection seam. This is strong contrast with the uphill picture used in transition state theory for thermal reactions, and it requires a new interpretation to understand and predict photochemical reaction outcomes.
Analog quantum simulation of photochemical dynamics
Quantum mechanical systems are difficult to simulate with conventional (classical) computers because the amount of information required to describe a quantum state scales exponentially with the system size. Photochemical reactions are especially difficult because they require a quantum mechanical treatment of atomic nuclei in addition to the electrons.
Quantum computing show promise as a platform for quantum chemical calculations at a lower computational cost; however, early digital quantum computers composed of quantum bits (qubits) are limited in size and number of operations due to environmental noise. In contrast, an analog quantum computer uses a controllable quantum system in a laboratory to simulate a system of interest, such as a molecule. With a suitable analog of a photoexcited molecule, a simulation consists of creating a desired initial state, then letting it evolve naturally with time and performing measurements.
Digital quantum simulation of photochemical properties
The last decade has shown great advances and ever-increasing interest in the development of quantum computers. One of the earliest foreseeable benefits of quantum computers is for the simulation of quantum mechanical systems, including quantum chemistry.
Most quantum chemistry methods, including those developed for future quantum computers, involve the Born-Oppenheimer approximation. It separates the equations for electrons and nuclei such that electrons behave as though the nuclei are fixed in place. This approximation has resulted in many efficient algorithms for the simulation of properties with classical computers, but it breaks down during photochemical dynamics involving internal conversion. Digital quantum computers give us the opportunity to re-think quantum chemistry methods with a more complete quantum mechanical description, with greater accuracy for nuclear quantum effects in photochemistry.
