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Subsections

[edit] Pseudopotentials

[edit] Supported format

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.

[edit] Obtaining a pseudopotential

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

[edit] Generating a pseudopotential

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.

[edit] References

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:


next up previous
Next: [edit] Using PWscf Up: User's Guide for Quantum-ESPRESSO Previous: [edit] Running on parallel
Paolo Giannozzi 2008-07-01