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--- package:faq [2022/04/01 11:41] – pbloechl
+++ package:faq [2024/03/04 08:42] (current) – pbloechl
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 The PAW method uses extensively the numerical methodology developed for the pseudopotential approach and for the LMTO method. This results in fast algorithms, and a close similarity between those methods.
-==== How do I obtain access to the CP-PAW code? ====
-Please note that the CP-PAW code is not easy to use, and requires some experience. Therefore, it is advisable if you have been trained in ab-initio molecular dynamics or if you have a colleague who is willing to help you during your first steps. It is certainly not the kind of code you want to use "on a side".
+==== I obtain negative occupations. Is that wrong? ====
-You require a license from Clausthal University of Technology to be able to access the code. There is a small fee for non-profit research organizations and an appropriate fee for commercial entities.
+No. This occurs normally for the tetrahedron method using the corrections from  [[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.49.16223|Blöchl et al. Phys. Rev. B 49, 16223 (1994)]]. It reflects that the Brillouin-zone integration is not approximated by a sampling over a discrete k-point set, but energies and matrix elements are interpolated in between. What is printed as occupations are actually integration weights, which include the interpolation between the discrete k-points. Using these elaborate integration weights, the integral of the interpolated matrix elements and energies can still be expressed as weighted sum over the discrete k-point set. The interpolation is effectively "hidden" from the user at the price of complicated integration weights, which take on values that appear, at first sight, unphysical. When the interpolation between the discrete k-points is non-linear, which is the case with the "correction formula" from [[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.49.16223|Blöchl et al. Phys. Rev. B 49, 16223 (1994)]], the integration weights may also be negative or larger than one. See for example Eq.22 and Fig.7 of [[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.49.16223|Blöchl et al. Phys. Rev. B 49, 16223 (1994)]].
-To request a license, please go to the PAW download page, where you will be asked to fill an electronic form with your contact details and you will find further details on the process.
-If you encounter problems, please contact
-      Prof. Dr. Peter E. Blöchl
-      Address:
-      Dept. of Applied Theoretical Physics
-      Institute of Theoretical Physics
-      Clausthal University of Technology
-      Leibnizstr. 10
-      D-38678 Clausthal-Zellerfeld
-      Germany
-      phone: +49(5323)722555
-      fax: +49(5323)723116
-      E-Mail: Peter.Bloechl@tu-clausthal.de