Electron density in ammonium dihydrogen phosphate: non-uniqueness of the multipolar model in simple inorganic structures

X-ray and neutron diffraction data of a single crystal of ammonium dihydrogen phosphate have been used for the determination of the electron density using multipolar expansion of the density around each nucleus. As the ammonium group was found to be nearly neutral from unconstrained multipole refinement, constrained refinements have been performed with the charge of the ammonium group ranging from zero to one. On the other hand, the expansion of the radial functions of the phosphorus atom was varied. All refinements led to almost the same agreement factors and residual densities. The consequences of such uncertainties on the topology of the electron density are discussed, namely the topology of the P-O bond critical point.