Display omitted •A new method for calculating the internal conversion rate constant is proposed.•The approach has been applied to the study of molecules important for organic electronics.•The method facilitates the design of efficient molecular systems. A new method for calculating the internal conversion rate constant, which takes into account all possible vibrational transitions between two adiabatic electronic states, is proposed. The simple analytical form of the correlation function obtained in the harmonic and anharmonic approximations and taking into account the Franck-Condon term makes it possible to perform fast calculations. In addition, the method takes into account temperature effects within harmonic kind, which extends its area of application to explain experimental processes. The approach has been applied to the study of molecules important for organic electronics, such as tetraoxa8circulene, free-base porphyrin and polynfluorenes. The method provides valuable information on IC processes and facilitates the design of efficient molecular systems.