# Espresso¶

Quantum ESPRESSO (QE) is an integrated suite of Open-Source computer codes for electronic-structure calculations and materials modeling at the nanoscale. It is based on density-functional theory, plane waves, and pseudopotentials.

The ASE calculator is an interface to the pw.x executable.

## Setup¶

Set up the calculator like a standard FileIOCalculator:

• export ASE_ESPRESSO_COMMAND="/path/to/pw.x -in PREFIX.pwi > PREFIX.pwo"

Any calculation will need pseudopotentials for the elements involved. The directory for the pseudopotential files can be set with the pseudo_dir parameter, otherwise QE will look in $ESPRESSO_PSEUDO if it is set as an environment variable if set; otherwise $HOME/espresso/pseudo/ is used. The pseudopotentils are assigned for each element as a dictionary:

pseudopotentials = {'Na': 'Na_pbe_v1.uspp.F.UPF',
'Cl': 'Cl.pbe-n-rrkjus_psl.1.0.0.UPF'}


A simple calculation can be set up:

from ase.build import bulk
from ase.calculators.espresso import Espresso
from ase.constraints import UnitCellFilter
from ase.optimize import LBFGS

rocksalt = bulk('NaCl', crystalstructure='rocksalt', a=6.0)
calc = Espresso(pseudopotentials=pseudopotentials,
tstress=True, tprnfor=True, kpts=(3, 3, 3))
rocksalt.calc = calc

ucf = UnitCellFilter(rocksalt)
opt = LBFGS(ucf)
opt.run(fmax=0.005)

# cubic lattic constant
print((8*rocksalt.get_volume()/len(rocksalt))**(1.0/3.0))


## Parameters¶

The calculator will interpret any of the documented options for pw.x: http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html

All parameters must be given in QE units, usually Ry or atomic units in line with the documentation. ASE does not add any defaults over the defaults of QE.

Parameters can be given as keywords and the calculator will put them into the correct section of the input file. The calculator also accepts a keyword argument input_data which is a dict, parameters may be put into sections in input_data, but it is not necessary:

input_data = {
'system': {
'ecutwfc': 64,
'ecutrho': 576}
'disk_io': 'low'}  # automatically put into 'control'

calc = Espresso(pseudopotentials=pseudopotentials,
tstress=True, tprnfor=True,  # kwargs added to parameters
input_data=input_data)


Some parameters are used by ASE, or have additional meaning:

• kpts=(3, 3, 3) sets the number of kpoints on a grid (defaults to gamma)

• koffset=(0, 0, 0) set to 0 or 1 to displace the kpoint grid by a half cell in that direction. Also accepts True and False.

• kspacing=0.1 sets the minimum distance between kpoints in reciprocal space.

• nspin=2 if any atom has a magnetic moment spin is turned on automatically.

Any FixAtoms or FixCartesian constraints are converted to Espresso constraints (for dynamic calculations).

## Alternative Calculators¶

There are several other QE Calculator implementations based on ase that provide a number of extra features:

## Espresso Calculator Class¶

class ase.calculators.espresso.Espresso(restart=None, ignore_bad_restart_file=False, label='espresso', atoms=None, **kwargs)[source]

All options for pw.x are copied verbatim to the input file, and put into the correct section. Use input_data for parameters that are already in a dict, all other kwargs are passed as parameters.

Accepts all the options for pw.x as given in the QE docs, plus some additional options:

input_data: dict

A flat or nested dictionary with input parameters for pw.x

pseudopotentials: dict

A filename for each atomic species, e.g. {'O': 'O.pbe-rrkjus.UPF', 'H': 'H.pbe-rrkjus.UPF'}. A dummy name will be used if none are given.

kspacing: float

Generate a grid of k-points with this as the minimum distance, in A^-1 between them in reciprocal space. If set to None, kpts will be used instead.

kpts: (int, int, int), dict, or BandPath

If kpts is a tuple (or list) of 3 integers, it is interpreted as the dimensions of a Monkhorst-Pack grid. If kpts is a dict, it will either be interpreted as a path in the Brillouin zone (*) if it contains the ‘path’ keyword, otherwise it is converted to a Monkhorst-Pack grid (**). (*) see ase.dft.kpoints.bandpath (**) see ase.calculators.calculator.kpts2sizeandoffsets

koffset: (int, int, int)

Offset of kpoints in each direction. Must be 0 (no offset) or 1 (half grid offset). Setting to True is equivalent to (1, 1, 1).

Note

Set tprnfor=True and tstress=True to calculate forces and stresses.

Note

Band structure plots can be made as follows:

1. Perform a regular self-consistent calculation, saving the wave functions at the end, as well as getting the Fermi energy:

>>> input_data = {<your input data>}
>>> calc = Espresso(input_data=input_data, ...)
>>> atoms.set_calculator(calc)
>>> atoms.get_potential_energy()
>>> fermi_level = calc.get_fermi_level()

2. Perform a non-self-consistent ‘band structure’ run after updating your input_data and kpts keywords:

>>> input_data['control'].update({'calculation':'bands',
>>>                               'restart_mode':'restart',
>>>                               'verbosity':'high'})

>>> bs = calc.band_structure()