Keywords: pseudo-gen

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## 1.1 Who needs to generate a pseudopotential?

There are at least three well-known published sets of NC-PP's: those of Bachelet, Hamann, and Schlüter [4], those of Gonze, Stumpf, and Scheffler [5], and those of Goedecker, Teter, and Hutter [6]. Moreover, all major packages for electronic-structure calculations include a downloadable table of PP's. One could then wonder what a PP generation code is useful for. The problem is that sometimes available PP's will not suit your needs. For instance, you may want:

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  a better accuracy;
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  PP's generated with some exotic or new exchange-correlation functional;
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  a different partition of electrons into valence and core;
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  ``softer'' PP's (i.e. PP that require a smaller cutoff in plane-wave calculations);
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  PP's with a core-hole for calculations of X-ray Adsorption Spectra;
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  all-electron wavefunctions reconstruction (requires the knowledge of atomic all-electron and pseudo-orbitals used in the generation of PP's);

or you may simply want to know what is a PP, how to produce PP's, how reliable they are.