NAD(+) is the oxidized form of the coenzyme redox pair NAD(+)/NADH and plays a main role in catalysis of biological processes. Accurate X-ray diffraction data have been collected at 100 K from a crystal of the free acid form of the coenzyme NAD(+) with a CCD area detector up to 0.55 Angstrom resolution. A multipole refinement was carried out, and the electron density of the molecule has been analyzed in order to further model its interaction with enzymes. The electron density of NAD(+) has also been computed by a DFr (density functional theory) approach. The charge density and electrostatic potential obtained from experiment and theory compare well. There is a very good agreement between the theoretical and experimental charge densities for all the C-C and C-N type connections of the NAD(+) molecule; this is less the case for the pyrophosphate region. The experimental electron density lone pairs of the pyrophosphate lateral oxygen atoms are distorted by hydrogen bonds effects. The electrostatic potential has large spatial extensions with highly positive and negative regions, suggesting that NAD(+) is likely to interact with proteins mostly by electrostatic interactions.