The long-standing problem of the origin of plasticity in (polymer) glasses still remains unsolved. Recently, detailed molecular simulations have unveiled a wealth of information concerning the (elastic) and plastic response of polymer glasses to deformation. By transforming the potential energy landscape of an atactic polystyrene specimen to a proper free energy landscape, we have thoroughly studied the response to deformation of local free energy minima and transition states (first-order saddle points). Local minima mostly shape the macroscopic thermo-elastic properties observed for glasses, e.g., moduli, while transition states dominate the dynamics (structural relaxation) of the glasses. By deforming the local free energy minima,1 the macroscopic stress-strain curve for polystyrene has been obtained (up to 100% compression – well into the plastic regime) and the evolution of the microscopic polymer configurations in the course of deformation has been studied. In order to analyse the latter, we have introduced a per-atom strain tensor that describes the best fit of a deformation field to a given atom and its first neighbours (defined by means of Voronoi discretization). The imposition of different strain levels onto the transition states2 allowed us to estimate the apparent activation volume of every elementary transition. Our findings are in good agreement with experimental observations for the same system, indicating that both length- and time-scales of the structural transitions of glassy aPS can be obtained by proper free-energy barrier-hopping of atomistically detailed configurations. The connection to macroscopic theories, e.g., the theory of activation by Eyring, is still elusive however several directions will be discussed. Acknowledgement: This research forms part of the research program of the Dutch Polymer Institute (DPI), projects #745ft14, #820 and #829.