In the present paper we introduce a new method to improve common force fields with respect to realistic modeling of the amorphous polymer bulk. To this end various force-field parameters are adjusted to results from ab initio calculations. Parameters for both intra- and intermolecular interactions are optimized for the first time using methods on an appropriate theoretical level. We derive a force field which is "tailor-made" for polypropylene and applicable to polypropylene chains of any tacticity. This new force field is used in subsequent molecular dynamics (MD) simulations of atactic polypropylene in a cubic box with periodic boundaries. We simulated an isothermal-isobaric (NpT) ensemble at p = 1 bar to predict characteristic bulk properties at temperatures above and below the glass point. Simulated densities and cohesive energies are in excellent agreement with experiment. Chain dimensions are characterized by the end-to-end distance and radius of gyration. The Pair correlation function calculated for glassy atactic polypropylene confirms. the complete absence of long-range order.