C-13/N-15 (experimental and ab initio) solid-state NMR was used to achieve an affordable way to improve hydrogen refinement of 6-aminopenicillanic acid (6-APA) structure. The lattice effect on the isotropic chemical shifts was probed by using two different magnetic shielding calculations: isolated molecules and periodic crystal structure. The electron density difference maps of optimized and non-optimized structures were calculated in order to investigate the interactions inside the 6-APA unit cell. The C-13 and N-15 chemical shifts assignments were unambiguously stablished. In addition, some of the literature C-13 resonances ambiguities could be properly solved. (C) 2016 Elsevier B.V. All rights reserved.