We present the first ab initio calculations of the internal magnetic field of a molecular magnet using a modified Ewald method. We apply this to both the -phase and -phase forms of the organic radical para-nitrophenyl nitronyl nitroxide (p-NPNN). The -phase is known to exhibit molecular ferromagnetism, whilst the -phase is antiferromagnetic. These results are of particular relevance to the interpretation of muon spin-rotation experiments, where the muon acts as a local probe of the internal magnetic field. As further guidance to the interpretation of such experiments, we also calculate the isotropic hyperfine constant for implanted muons.