This work concerns a comparison of experimental and theoretical results of the electron charge density distribution and the electrostatic potential around the m-nitrophenol molecule (m-NPH) known for its interesting physical characteristics. The molecular experimental results have been obtained from a high-resolution X-ray diffraction study. Theoretical investigations were performed using the Density Functional Theory at B3LYP level of theory at 6-31G* in the Gaussian program. The multipolar model of Hansen and Coppens was used for the experimental electron charge density distribution around the molecule, while we used the DFT methods for the theoretical calculations. The electron charge density obtained in both methods allowed us to find out different molecular properties such us the electrostatic potential and the dipole moment, which were finally subject to a comparison leading to a good match obtained between both methods. The intramolecular charge transfer has also been confirmed by an HOMO-LUMO analysis.

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m-Nitrophenol (m-NPH) occurs at room temperature in two solid modifications: monoclinic P21/n and orthorhombic P212121 [Pandarese, Ungaretti & Coda (1975).Acta Cryst. B31, 2671-2675]. The thermal vibrations and electronic-density distribution of the molecule in the orthorhombic structure have been analyzed in terms of Stewart's rigid pseudoatom model, using restricted Slater radial functions and angular multipole terms extending to actapoles for C, N and O and dipoles for H pseudoatoms. The net atomic charge and the in-crystal molecular dipole moment have been determined in order to understand the nature of the inter-and intramolecular charge transfer. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of the H atoms, in the absence of complementary neutron diffraction data. ExperimentalThe purification of the material and the crystal growing were published earlier (Wojcik & Marqueton, 1989) and the experimental details and crystal data are given in Table 1. X-ray intensity data were measured on a CAD-4 diffractometer using graphite-monochromated MoKt~ radiation (50kV, 35 mA). A crystal of high quality was cooled by a stream of cold nitrogen gas. Lattice constants were determinated by a least-squares fit of setting angles for 25 well centered reflections in the range 11 _< 0 _< 25 °. The profiles of the different reflections were measured using the 0-20 step scan method. Three standard reflections were measured every 2h, the corresponding linear decay correction was performed but nonsignificant variation was observed and the obtained instrumental factor was 0.015. The intensities were measured only once with a relatively slow speed of 1.02 ° min -~. A total of 2224 intensities were collected and merged to give 2004 independent reflections. The internal agreement for all equivalent reflections was 0.02 (in reality only the standard references were measured more times). As a preliminary check on the X-ray intensity data, the atomic positions were obtained by a

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