Spectrum simulation and decomposition with nuclear ensemble: formal derivation and application to benzene, furan and 2-phenylfuran

A formal derivation of the nuclear-ensemble method for absorption and emission spectrum simulations is presented. It includes discussions of the main approximations employed in the method and derivations of new features aiming at further developments. Additionally, a method for spectrum decomposition is proposed and implemented. The method is designed to provide absolute contributions of different classes of states (localized, diffuse, charge-transfer, delocalized) to each spectral band. The methods for spectrum simulation and decomposition are applied to the investigation of UV absorption of benzene, furan, and 2-phenylfuran, and of fluorescence of 2-phenylfuran.