A little historic background of ORTEP & friends
In any given crystalline solid, all the atoms are moving all the time, and the amount of movement increases with the temperature. Even at absolute zero, however, there is atomic movement due to Heisenberg├ó┬?┬?s famous uncertainty principle. The fact that such atomic movement also affects the intensities in an X-ray (and neutron) diffraction experiment was first realized by Peter Debye ( Ann. Phys. 1913, 348, 49) and further studied by Ivar Waller ( Z. Phys. 1923, 17, 398) ├ó┬?┬? hence the historic ├ó┬?┬?Debye-Waller factor├ó┬?┬Ł for correcting an atom├ó┬?┬?s scattering power by a temperature-dependent term.
With the advent of digital computing in the 1960s, refining a crystallographic model based on X-ray (and neutron) intensities became rather straightforward, at least in principle (not necessarily in practice), and people soon also modelled the amount of atomic movement by a refinable ├ó┬?┬?atomic displacement parameter├ó┬?┬Ł (ADP) as it is called using modern nomenclature. Because atomic movement is not necessarily isotropic, refinable anisotropic ADPs were the next step, typically expressed by a symmetric second-rank tensor (that is, a symmetric 3 x 3 matrix composed of real numbers). Because the latter is pretty hard to imagine, a landmark contribution was C. K. Johnson├ó┬?┬?s ORTEP (Oak Ridge Thermal Ellipsoid Plot) computer program published in 1965 which managed to visualize the aforementioned numerical entries. Yes, this website├ó┬?┬?s logo starts with such an ORTEP plot because we can eventually see how the atoms vibrate ├ó┬?┬? and zillions of ORTEP plots drawn (and published) up to the present day with whatever kind of software tells you something about the ingenuity of Johnson├ó┬?┬?s approach.
last modified: 2015-10-23