AB-INITIO STUDIES OF MAGNETISM IN THE ORGANIC RADICAL P-NPNN

We present the first ab initio calculations of the internal magnetic f ield of a molecular magnet using a modified Ewald method. We apply thi s to both the beta-phase and gamma-phase forms of the organic radical para-nitrophenyl nitronyl nitoride (p-NPNN). The beta-phase is known t o exhibit molecular ferromagnetism, whilst the gamma-phase is antiferr omagnetic. These results are of particular relevance to the interpreta tion of muon spin-rotation (mu(+)SR) experiments, where the muon acts as a local probe of the internal magnetic held. As further guidance to the interpretation of such experiments, we also calculate the isotrop ic hyperfine constant for implanted muons.