Interesting proton behaviour in molecular structures. Variable temperature neutron diffraction and ab initio study of acetylsalicylic acid: characterising librational motions and comparing protons in different hydrogen bonding potentialsElectronic supplementary information (ESI) available: bond lengths and angles from the refinements, refined atomic coordinates and ADPs. See http://www.rsc.org/suppdata/nj/b2/b203775k/