Pseudo potentials
In Quantum Espresso, pseudopotential replaces the actual electronion 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.
You may try my Pseudos WebApp, where you can look for pseudopotentials by element name, and download them. Currently, Standard Solid State Pseudopotentials (SSSP) and GBRV pseudopotentials are included.
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 spinorbit 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:
 https://www.quantumespresso.org/pseudopotentials
 https://www.materialscloud.org/discover/sssp/table/efficiency
 http://www.pseudodojo.org
 https://www.physics.rutgers.edu/gbrv/
 https://nninc.cnf.cornell.edu
 http://www.quantumsimulation.org/potentials/
 BLYP pseudopotentials
 SCAN pseudopotentials
Pseudopotential naming conventions in PSLibrary: an example pseudopotential
filename is O.relpbenrrkjus_psl.1.0.0.UPF
.
O
→ denotes the atomic species
rel
→ full relativistic (optional)
pbe
→ exchange correlation functional
n
→ nonlinear core correction (optional)
rrkjus
→ pseudopotential type
Exchange correlation functionals:
Identifier  Functional 

pz  PerdewZunger (LDA) 
pbe  PerdewBurkeErnzerhof (GGA) 
pw91  PerdewWang 91 (GGA) 
blyp  BeckeLeeYangParr (GGA) 
Pseudopotential types:
Identifier  PP types 

ae  allelectron 
rrkj  RappeRabeKaxirasJoannopoulos (Norm conserving) 
rrkjus  RappeRabeKaxirasJoannopoulos (Ultrasoft) 
kjpaw  KresseJoubert (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 ultrasoft
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.

We can mix different types of pseudo potentials (e.g., norm conserving, ultrasoft, 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. 
"sol" in PBEsol stands for solid. For bulk systems PBEsol should be used, while PBE is appropriate for molecules. In case of 2D materials generally PBE is chosen, but one can check PBEsol.
If you mix PBE with PBEsol 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.