A quasi-harmonic molecular-mean-field model for analyzing anharmonic temperature evolution of anisotropic displacement parameters is described. Anharmonic effects are taken into account through a Gruneisen-type temperature dependence of effective vibrational frequencies. The method is applied to neutron and X-ray diffraction data of hexamethylenetetramine measured between 15 and 298 K. The resulting Gruneisen parameters and other characteristics of molecular motion in the solid state agree well with those obtained from independent vibrational data. The analysis also suggests errors in the ADP's due to insufficient extinction corrections in the diffraction data.