We present a detailed comparison of the segmental and chain dynamics of an atactic monodisperse polystyrene (molecular weight 1800 g/mol) (a-PS) studied by hierarchical multiscale molecular dynamics (MD) simulations, with an atactic polystyrene (M-n = 1644 g/mol, polydispersity index 1.14) investigated by dielectric spectroscopy, rheology and differential scanning calorimetry. The MD simulations, performed at three temperatures (403, 433, and 463 K), can capture the temperature dependence of the segmental and terminal relaxations in good quantitative agreement with experiment, taking into account the uncertainties in the development of the atomistic force field. In addition, ring and backbone segmental dynamics are studied by analyzing time-correlation functions for various bond vectors in the monomer level. MD simulations at elevated pressures (40 and 60 MPa) were also in good agreement with experiments probing the pressure-dependent glass temperature.