A new methodology to calculate efficiently the absorption spectrum of a single molecule when subjected to mechanical stress is presented. As example, the developed methodology was applied to cis- and trans-azobenzene, commonly used as photoswitch in a wide variety of applications. The results show that both (1)(n,π*) and (1)(π,π*) optical transitions can be efficiently modulated by applying an external force. A structural analysis was performed to evaluate the role of each internal coordinate in the excitation process, taking into account the application of external forces at different positions of azobenzene. Moreover, stress-strain curves were calculated in order to determine the maximum applicable forces within the elastic region, highlighting notable differences between the mechanical properties of cis- and trans-azobenzene conformers. The optomechanical work obtained by elongation and compression steps is calculated for a single azobenzene molecule and compared to available experimental data. Finally, the implications derived from the application of azobenzene as main chain component of a linear polymer acting as a photoinduced motor are discussed.