NCrystal release 2.6.0

• Introduce NCMAT v4 which introduces a few new features, as described in detail on the NCMAT-format page. The most important new feature relies on the fact that it is possible to estimate atomic mean-squared-displacements directly from a phonon density of state distribution (VDOS), meaning that it is now allowed (and actually recommended) to skip @DEBYETEMPERATURE's for crystalline materials where atoms have VDOS curves available). This is not only useful, but improves low-temperature reliability of the Debye-Waller factors used in the elastic scattering components. This development was helped by Kemal Ramic (@ramic-k) and Jose Ignacio Marquez Damian (@marquezj) from the ESS Spallation Physics group, and more details are available on the related issue page (#58). The other new feature in NCMAT v4 is a new optional syntax for the @CELL section, which allows more compact specification for certain crystal systems (in particular cubic systems).
• The vast majority of crystalline materials in the NCrystal data library were updated to take advantage of the new features, and in addition the number of VDOS curves was increased dramatically, meaning that the dynamics in almost all crystalline materials are now described using actual VDOS curves rather than just Debye temperatures. The Data-library page of the wiki has been updated accordingly, and in brief the modified materials are:
  • Missing VDOS curves for all elements in beryllium-fluoride (BeF2), aragonite (CaCO3), cuprite (Cu2O), periclase (MgO), and quartz (SiO2) were provided by Kemal Ramic from the ESS Spallation Physics group, based on modelling with Phonopy, VASP and OCLIMAX software.
  • Missing VDOS curves for 14 mono-atomic materials were added (fixing issue #39): Au, Ba, Cr, Mg, Mo, Na, Nb, Pd, Pt, Rb, Sc, Sn, Y, and Zn. Refer to comments in the individual NCMAT files for references concerning the origin of these curves.
  • Missing VDOS curves were added to uranium oxide (UO2), taken from DOI:10.1103/PhysRevB.102.134312. Note that UO2 NCMAT files with VDOS tuned for specific temperatures are available in the ncrystal-extra repo.
  • All existing files with VDOS have been update to NCMAT v4, and had @DEBYETEMPERATURE sections removed in order to benefit from the better performance of getting the atomic displacements from the VDOS.
  • Added new material: gamma-Iron (Fe_sg225_Iron-gamma.ncmat) with VDOS curve. At the same time, the not particularly useful beta-Iron file was removed, so the NCrystal data library now contains both alpha- and gamma-iron, both with VDOS curves.
• A few other issues were addressed, for details refer to the CHANGELOG.