The energetics and structure of the lowest-energy polymorphic forms of the acetic acid crystal previously found by using the UPACK polymorph predictor program and standard atom--atom potentials (GROMOS) has been evaluated by a first-principles Car--Parrinello molecular dynamics method, which takes into account the periodic conditions of the solids within the framework of density functional (DFT) methodology. Our results show that one has to include a correction on the DFT energy to reproduce the experimental value of the interaction energy of the crystal, although this correction does not change the relative energetic ordering of the most stable polymorphic forms. It is also found that the most stable form in these ab initio computations is the UPACK/GROMOS structure closest to the experimental one (polymorph 13), which ranked 15th in the UPACK/GROMOS computations. The crystal structures obtained by the UPACK/GROMOS are qualitatively sounding, but they are too densely packed when compared to the experimental structure. Due to this fact, the DFT interaction energy of these forms is much weaker than that computed for the experimental structure. Due to this, when the UPACK/GROMOS structure of polymorph 13 is fully optimized at the ab initio level, it expands and transforms without barrier to the low-pressure experimental structure. A short-time ab initio dynamics simulation at 150 K on that relaxed structure did not show the presence of phase transformation toward another polymorphic structure. A similar trend is found for the high-pressure polymorph.