FAQ

Running espresso

• How do I initialize a self-consistent calculation with the Kohn-Sham potential from a previous calculation?
  Store the charge density with calc.save_chg(filename) and reload it for the new calculator with othercalc.load_chg(filename). Use startingpot='file' in the new calculator definition to initialize the Kohn-Sham potential from the loaded charge density.

• How do I initialize a non-self-consistent calculation with the Kohn-Sham potential from a previous calculation?
  Preferably use save_flev_chg/load_flev_chg so that the Fermi level E_F is stored in addition to the charge density, and band structure or density of states can be plotted with respect to E_F. See the Band structure calculation example.

Things to consider when switching from Dacapo to Espresso

• Density cut-offs are defined differently in Dacapo and Espresso; when using ultrasofts the rule of thumb for Espresso is dw=10pw (when only norm-conserving pseudopotentials are used dw must be 4pw.)
• Dacapo will turn on k-point parallelization automatically, use parflags='-npool number_of_nodes' to turn on k-point-/spin-parallelization in Espresso (make sure you have enough k-points to distribute (evenly)).
• With equivalent parameters and when using the same ultrasofts, total energies from Dacapo and Espresso can be compared. Note, however, that the default MP k-point grids of the form n₁xn₂xn₃ with any n_i even are different because Espresso centers at Γ (unless a non-zero kptshift is specified), while Dacapo does not perform centering.
• Espresso's default charge-density mixing parameter is large (70%). Espresso's mixer is actually able to handle this and it generally works well for bulk systems, but for slab calculations 30% is a safer choice (convergence={'mixing':0.3}; adding a preconditioner via 'mixing_mode':'local-TF' can further improve convergence for slab calculations).
• Default pseudopotentials are not all identical to Dacapo ones; make sure to check the minimum recommended cutoffs and run convergence tests accordingly.

Using the interface to only create input for pw.x to run pw.x separately without the ase-espresso interface

• The following snippet will setup a simple pw.inp file (with most parameters at their default values)

from espresso import espresso
from ase.lattice import bulk
atoms = bulk('Pt','fcc',4.0)
calc = espresso(pw=400,nbands=24,kpts=(12,12,12),mode='scf',onlycreatepwinp='pwscf.inp')
calc.initialize(atoms)

• Constraints on coordinates supported by Quantum Espresso are translated from ase constraints.
• More complex contraints require pw.x to be run by ase-espresso, which requires espsite.py to be configured for the computers used (there are examples for this file for several schedulers in the git checkout).