Anharmonic Refinement
It is possible to refine the displacements of selected atoms anharmonically in olex2.refine. Here’s how it is done.
OLEX2 This video is somewhat out of date – third and fourth order harmonics are also supported now.
Warning: This is an experimental feature – it works, but please use it at your own disgression. You must be aware that this technique is open to abuse.
Select the atom(s) you want to refine anharmonically and type anis -a=3 for third order or anis -a=4 for fourth order harmonics. If you don’t want to refine atoms anharmonically any longer, please select these atoms (or all!) and just type anis – this will remove the anharmonic flag for all.
After the refinement, the PDF (probability density funcion) will be displayed around the atoms that have been refined anisotropically. CTL+M will hide this display (or RIGHT CLICK on the mesh).
You can also visualise the resulting ‘shape’ in terms of mean square displacement of the atoms by switching on an alternative display style for all atoms, type MSDSView -a=anh -s=1.5 Please keep in mind that the scale of 1.5 will apply and that for the anharmonic atoms only the contribution of the anharmonic terms is shown. You can disable this view by typing kill MSDS.
A judgement whether it might be reasonable to refine anharmonic motion can be made according to Kuhs' rule: Kuhs Aust. J. Phys. 1988,41,369-82. 0.074758 sqrt(n/U^2) might give an estimate what resolution in sinus theta over lambda is required to have a significant difference between anharmonic and harmonic motion.