APD-Toolkit and the Invariom+TLS approach for estimating H-ADPs
What is it about?
Hydrogen ADPs can now be estimated with a new stand-alone program called APD-Toolkit. ADPs have contributions from lattice vibrations and molecular vibrations. The former are obtained from a TLS fit to the non-hydrogen ADPs, the latter are taken from quantum chemical geometry optimizations of model compounds. Both are combined to give ADPs for H-atoms. We have, over the years, built a database of suitable model compounds called the "invariom" database. It also contains aspherical scattering factors.
Why is it important?
Modelling the anisotropic displacement parameters also for hydrogen atoms is especially important for charge density studies, where the aim is to get an accurate electron density distribution for a molecule in the crystal lattice. Since X-ray single-crystal diffraction usually does not allow to refine H-ADPs due to their weak scattering contribution information on their displacements has to come from another source, e.g. neutron diffraction. But neutron experiments are non-routine so we need something more straight-forward.