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Pseudo potentials

In Quantum Espresso, pseudopotential replaces the actual electron-ion interaction. The pseudopotential describes the atomic nucleus and all the electrons except the outermost valence shell. The rapidly changing potential field near the atomic core is replaced by a smoother function that simulates the potential field far from the core very well. By doing so, it requires less number plane wave basis for wavefunction expansion.

We can choose form various pseudopotential libraries. Choice of pseudopotential depends on the problem we are investigating, e.g., if there is a heavy element present in our system and we are interested in the spin-orbit coupling effects, we should choose a full relativistic pseudopotential. We need to be careful whether our chosen pseudopotential correctly reproduces physical properties. Various pseudopotential libraries:

Pseudopotential naming conventions in PSLibrary: an example pseudopotential filename is O.rel-pbe-n-rrkjus_psl.1.0.0.UPF.

O → denotes the atomic species rel → full relativistic (optional) pbe → exchange correlation functional n → non-linear core correction (optional) rrkus → pseudopotential type

Exchange correlation functionals:

pzPerdew-Zunger (LDA)
pbePerdew-Burke-Ernzerhof (GGA)
pw91Perdew-Wang 91 (GGA)
blypBecke-Lee-Yang-Parr (GGA)

Pseudopotential types:

IdentifierPP types
rrkjRappe-Rabe-Kaxiras-Joannopoulos (Norm conserving)
rrkjusRappe-Rabe-Kaxiras-Joannopoulos (Ultrasoft)
kjpawKresse-Joubert (PAW)

Ultra soft pseudopotentials are computationally efficient than the norm conserving pseudopotentials. You will find the recommended ecutwfc in the header of each pseudopotential file. If you choose an ultra-soft pseudopotential, you will need ecutrho about 8 times the value of ecutwfc. The default ecutrho is 4 times ecutwfc in Quantum Espresso code, which is a good choice for norm conserving pseudopotentials. You should check energy convergence against ecutwfc for your system.

By using pseudopotential, we want to get rid of the core electrons that do not participate in the chemical properties of material. This is known also as rigid core approximation. Instead of accounting the nucleus and core electrons separately, we want to have a pseudopotential that interacts in a similar way with the valence electrons.

  1. We can mix different types of pseudo potentials (e.g., norm conserving, ultra-soft, or PAW), but we cannot mix different exchange correlation functional (e.g., PBE and LDA). Exchange correlation functional can be read from the pseudopotential file or be provided via input_dft parameter in Quantum Espresso.

  2. "sol" in PBE-sol stands for solid. For bulk systems PBE-sol should be used, while PBE is appropriate for molecules. In case of 2D materials generally PBE is chosen, but one can check PBE-sol.

Common error

If you mix PBE with PBE-sol type, it results in Error: conflicting values for igcx. However, it is allowed to mix those two types of pseudo. We can set desired exchange correlation functional via input_dft instead of reading from the pseudopotential file.