An atomistic computer simulation was performed for the polyimides ULTEM™ and EXTEM™ via the molecular-dynamics method with the use of Gromos53a6 and Amber99 force fields. For parameterization of electrostatic interactions, the partial atomic charges were calculated through quantum-chemical methods. The temperature dependence of density and the thermal-expansion coefficients for the polyimides were obtained. The calculated density values of the polyimides at room temperature and their coefficients of thermal expansion in the glassy state are in agreement with available experimental data. It is shown that inclusion of electrostatic interactions is necessary for simulation of the thermophysical characteristics of the considered polyimides.