Final Activity Report Summary - MMVBSCD (Semi-classical dynamics on large systems using the MMVB method: Applications to photochemical switches and electron transfer systems) The low-energy excitations determine the physical properties and chemical reactivity of a vast number of molecular systems. The spin Hamiltonian valence bond approach allows for an efficient treatment of large active spaces. When combined with molecular mechanics the resulting hybrid method (molecular mechanics - valence bond, MMVB) becomes a unique tool for studying photochemistry of large molecules. The method was originally developed only for the limiting case of covalent states and active spaces with equal numbers of electrons and orbitals.The main result of the fellowship is the extension of the MMVB method to the general active spaces without any restrictions on the numbers of electrons and orbitals. Also, the new method allows considering the charge-transfer excited states. Both developments significantly extend the applicability of the method. The resulting magnetic Hamiltonians have been partially parameterised and all the algorithms have been implemented allowing for a detailed analysis of potential energy surfaces. The practical applications explaining the photostability and relaxation mechanism of pyrene radical cation and fluorescence of perylene radical cation as well as properties of other polycyclic aromatic hydrocarbon cations demonstrate the high potential of the extended MMVB method in theoretical studies of photochemical behaviour.
