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 ==== What is the PAW Method? ====
-The Projector Augmented Wave method is a simulation algorithm to obtain the total energy, electron distribution and atomic structure and dynamics. The term PAW is often also used as short hand for the CP-PAW code developed originally by Peter Blöchl.
+The Projector Augmented Wave (PAW) method is a simulation algorithm to obtain the total energy, electron distribution and atomic structure and dynamics. The term PAW is often also used as short hand for the CP-PAW code developed originally by Peter Blöchl.
 ==== Where can I find a description of the PAW method? ====
-The original reference is Peter E. Blöchl, Phys. Rev. B 50, 17953 (1994). Some of the ideas have been shaped during earlier pseudopotential work described in Peter E. Blöchl, Phys. Rev. B 41, 5414 (1989). There is also a publication focusing on people new to PAW: "The Projector augmented wave method: ab-initio molecular dynamics with full wave functions"
+The original reference is [[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.50.17953|P.E. Blöchl, Phys. Rev. B 50, 17953 (1994)]]. Some of the ideas have been shaped during earlier pseudopotential work described in [[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.41.5414|P.E. Blöchl, Phys. Rev. B 41, 5414 (1989)]]. There is also a publication targeting beginners to PAW: [[https://link.springer.com/article/10.1007/BF02712785|P.E. Blöchl, C.J. Först  and J.Schimpl, Bull. of Mater. Sci. 26, 33 (2003)]] [[https://arxiv.org/abs/cond-mat/0201015|(Arxiv Version)]].
-Peter E. Blöchl, Clemens J. Först and Johannes Schimpl, submitted to Bull. Mater. Sci. (Dec. 31, 2001)
-Here is a introductory presentation on ab-initio methods and here a presentation specificly on the PAW Method given by P. Bloechl at the CAMD Summer School 2010.
-Are there other implementations of PAW besides the CP-PAW code?
-I am aware of a view other implementations of the PAW method
+==== Are there other implementations of the PAW method ====
+The PAW method is used by a number of electronic structure codes. (The list is probably not complete)
   * [[https://abinit.org|ABINIT]]
   * [[http://www.castep.org|CASTEP]]
   * [[https://wiki.fysik.dtu.dk/gpaw/|GPAW]]
+  * [[https://cppaw.org|CPPAW]]
   * [[https://nwchemgit.github.io/Plane-Wave-Density-Functional-Theory.html|NWChem]]
   * [[https://www.onetep.org|ONETEP]]
-  * PWPAW
+  * [[http://users.wfu.edu/natalie/papers/pwpaw/man.html|PWPAW]]
     * N. A. W. Holzwarth, G. E. Matthews, R. B. Dunning, A. R. Tackett, and Y. Zen g, Phys. Rev. B 55, 2005 (1997).
     * Computer Physics Communications 135 329-347, 348-376 (2001)
+  * [[https://dft.sandia.gov/socorro/|Socorro]]
   * [[https://sxrepo.mpie.de/projects/sphinx/wiki|S/PHI/nX]]
   * [[https://www.quantum-espresso.org|Quantum Espresso]]
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-Another branch of methods uses the reconstruction of the PAW method, without taking into account the full wave functions in the self-consistency. Following chemist notation this approach could be termed "post-pseudopotential PAW". This development began with the evaluation for hyperfine parameters from a pseudopotential calculation using the PAW reconstruction operator (Van de Walle, C.G. and Blöchl, P.E., 1993, Phys. Rev. B 47, 4244) and is now used in the pseudopotenital approach to calculate properties that require the correct wave functions.
+Besides complete implementations of the PAW method, there is another branch of methods using the reconstruction of wave functions from the PAW method for property calculations, but not during the self-consistency cycle. This approach could be termed "post-pseudopotential PAW". This development began with the evaluation for hyperfine parameters from a pseudopotential calculation using the PAW reconstruction operator ([[https://journals.aps.org/prb/abstract/10.1103/PhysRevB.47.4244|C.G. Van de Walle and P.E. Blöchl, Phys. Rev. B 47, 4244 (1993)]]) and is now used in the pseudopotential approach to calculate properties that require the correct wave functions.
 ==== What is the relation between PAW and the pseudopotential approach? ====
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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".
-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.
-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.
+==== I obtain negative occupations. Is that wrong? ====
-If you encounter problems, please contact
+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)]].
-      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