This thesis deals with the excited-states dynamics of molecules beyond the Bon-Oppenheimer approximation (nonadiabatic dynamics). The nuclei were always treated with classical mechanics while the electrons were treated quantum mechanically, using linear-response time-dependent density functional theory for the calculation of electronic excited states. The focus of the work is the treatment of intersystem crossing through the use of auxiliary wavefunctions to calculate spin-orbit coupling. \ The text includes a review of the necessary theoretical concepts, an description of the method used to calculate spin-orbit coupling and applications to static calculations and dynamics simulations of small molecules and large carbon nanostructures.