A vibrationally accurate potential energy function, our spectroscopically determined force field (SDFF), is used to calculate the relative zero-point-corrected energies of various forms I-IV syndiotactic polypropylene (sPP) crystal structures, both in their unconstrained and room-temperature unit cells. The results confirm the instability of the pure Ibca structure and support suggestions that form I sPP may be a statistical mixture of structures. Form IV is shown to have an energy close to form I structures, making its participation in kink bands possible. The relatively high energy of the form III unit cell makes it understandable that this structure is found only under external tension.