Currently PWscf and CP support both Ultrasoft (US) Vanderbilt pseudopotentials (PPs) and Norm-Conserving (NC) Hamann-Schluter-Chiang PPs in separable Kleinman-Bylander form. Calculation of third-order derivatives is not (yet) implemented with US PPs. The Gamma-only phonon code is also restricted to NC PPs.
The Quantum-ESPRESSO package uses a unified pseudopotential format (UPF [1]) for all types of PPs, but still accepts a number of other formats:
Support for the “old CP” and “old FPMD” formats for NC PPs (CP only) is discontinued in v.4 and later. See also http://www.pwscf.org/oldformat.htm.
A large collection of PPs (currently about 75 elements covered) can be downloaded from the Pseudopotentials Page of the Quantum-ESPRESSO web site (http://www.pwscf.org/pseudo.htm). Check the naming convention for the pseudopotential files.
Other electronic-structure packages provide tables of PPs that can be freely downloaded, but need to be converted into a suitable format for use with Quantum-ESPRESSO:
Binary Vanderbilt format, can be converted using the uspp2upf.x utility in the upftools/
directory. In case of trouble with binary files read this:
http://www.theochem.ruhr-uni-bochum.de/ axel.kohlmeyer/cpmd-linux.html#endian
Some of these pseudopotentials can be converted using the fhi2upf.x utility in the upftools/ directory
If you do not find there the PP you need (because there is no PP for the atom you need or you need a different exchange-correlation functional or a different core-valence partition or for whatever reason may apply), it may be taken, if available, from published tables, such as e.g.:
or otherwise it must be generated. Since version 2.1, Quantum-ESPRESSO includes a PP generation package, in the directory atomic/ (sources) and atomic_doc/ (documentation, tests and examples). The package can generate both NC and US PPs in UPF format. We refer to its documentation for instructions on how to generate PPs with the atomic/ code.
Other PP generation packages are available on-line:
The first two codes produce PPs in one of the acceptable formats; the third, in a format that can be converted to unified format using the utilities of directory upftools/.
Remember: always test the PPs on simple test systems before proceeding to serious calculations.
This section contains an extended list of references for NC PPs.
Here is the very basic and essential:
The second reference discusses the fully seperable Kleinman-Bylander (KB) form of the NC PP. This is what is implemented in Quantum Espresso and most PW-based DFT codes. The reference below contains a more comprehensive discussion and is highly recommended.
It is often necessary to include unbound high angular momentum states in the NC PP. For example, d states in silicon. D. R. Hamann generalized this procedure to states of arbitrary energy:
The construction of the NC PP within the r_c is not unique and so there are several prescriptions with that of Troullier-Martins (TM) and Rappe-Rabe-Kaxiras-Joannopoulos (RRKJ) being the most frequently used:
When there is a significant overlap of the valence and core charge density, it is often necessary to use nonlinear core corrections (NLCC):
Finally here are advanced topics in NC PP, mostly for the very ambitious: