VASP

Introduction

VASP is a density-functional theory code using pseudopotentials or the projector-augmented wave method and a plane wave basis set. This interface makes it possible to use VASP as a calculator in ASE, and also to use ASE as a post-processor for an already performed VASP calculation.

Note

A new VASP calculator is currently in BETA testing, see vasp2, which implements the calculator using the FileIOCalculator.

Environment variables

You need to write a script called run_vasp.py containing something like this:

import os
exitcode = os.system('vasp')

The environment variable VASP_SCRIPT must point to that file.

A directory containing the pseudopotential directories potpaw (LDA XC) potpaw_GGA (PW91 XC) and potpaw_PBE (PBE XC) is also needed, and it is to be put in the environment variable VASP_PP_PATH.

Set both environment variables in your shell configuration file:

$ export VASP_SCRIPT=$HOME/vasp/run_vasp.py
$ export VASP_PP_PATH=$HOME/vasp/mypps

The following environment variable can be used to automatically copy the van der Waals kernel to the calculation directory. The kernel is needed for vdW calculations, see VASP vdW wiki, for more details. The kernel is looked for, whenever luse_vdw=True.

$ export ASE_VASP_VDW=$HOME/<path-to-vdw_kernel.bindat-folder>

The environment variable ASE_VASP_VDW should point to the folder where the vdw_kernel.bindat file is located.

VASP Calculator

The default setting used by the VASP interface is

class ase.calculators.vasp.Vasp(restart=None, output_template='vasp', track_output=False, **kwargs)[source]

Below follows a list with a selection of parameters

keyword type default value description
restart bool None Restart old calculation or use ASE for post-processing
xc str ‘PW91’ XC-functional. Defaults to None if gga set explicitly.
setups str None Additional setup option
pp str Set by xc or gga Pseudopotential (POTCAR) set used (LDA, PW91 or PBE).
kpts various \(\Gamma\)-point k-point sampling
gamma bool None \(\Gamma\)-point centered k-point sampling
reciprocal bool None Use reciprocal units if k-points are specified explicitly
prec str   Accuracy of calculation
encut float   Kinetic energy cutoff
ediff float   Convergence break condition for SC-loop.
nbands int   Number of bands
algo str   Electronic minimization algorithm
ismear int   Type of smearing
sigma float   Width of smearing
nelm int   Maximum number of SC-iterations

For parameters in the list without default value given, VASP will set the default value. Most of the parameters used in the VASP INCAR file are allowed keywords. See the official VASP manual for more details.

Note

Parameters can be changed after the calculator has been constructed by using the set() method:

>>> calc.set(prec='Accurate', ediff=1E-5)

This would set the precision to Accurate and the break condition for the electronic SC-loop to 1E-5 eV.

Exchange-correlation functionals

The xc parameter is used to define a “recipe” of other parameters including the pseudopotential set pp. It is possible to override any parameters set with xc by setting them explicitly. For example, the screening parameter of a HSE calculation might be modified with

>>> calc = ase.calculators.vasp.Vasp(xc='hse06', hfscreen=0.4)

The default pseudopotential set is potpaw_PBE unless xc or pp is set to pw91 or lda.

xc value Parameters set
lda, pbe, pw91 pp (gga set implicity in POTCAR)
pbesol, revpbe, rpbe, am05 gga
tpss, revtpss, m06l metagga
vdw-df, optpbe-vdw gga, luse_vdw, aggac
optb88-vdw, obptb86b-vdw gga, luse_vdw, aggac, param1, param2
beef-vdw gga, luse_vdw, zab_vdw
vdw-df2 gga, luse_vdw, aggac, zab_vdw
hf lhfcalc, aexx, aldac, aggac
pbe0 gga, lhfcalc
b3lyp gga, lhfcalc, aexx, aggax, aggac, aldac
hse03, hse06, hsesol gga, lhfcalc, hfscreen

It is possible for the user to temporarily add their own xc recipes without modifying ASE, by updating a dictionary. For example, to implement a hybrid PW91 calculation:

from ase.calculators.vasp import Vasp
Vasp.xc_defaults['pw91_0'] = {'gga': '91', 'lhfcalc': True}

calc = Vasp(xc='PW91_0')

Note that the dictionary keys must be lower case, while the xc parameter is case-insensitive when used.

Setups

For many elements, VASP is distributed with a choice of pseudopotential setups. These may be hard/soft variants of the pseudopotential or include additional valence electrons. Three base setups are provided:

minimal (default):
If a PAW folder exists with the same name as the element, this will be used. For the other elements, the PAW setup with the least electrons has been chosen.
recommended:
corresponds to the table of recommended PAW setups supplied by the VASP developers.
gw:
corresponds to the table of recommended setups for GW supplied by the VASP developers.

Where elements are missing from the default sets, the Vasp Calculator will attempt to use a setup folder with the same name as the element. A default setup may be selected with the setups keyword:

from ase.calculators.vasp import Vasp
calc = Vasp(setups='recommended')

To use an alternative setup for all instances of an element, use the dictionary form of setups to provide the characters which need to be added to the element name, e.g.

calc = Vasp(xc='PBE', setups={'Li': '_sv'})

will use the Li_sv all-electron pseudopotential for all Li atoms.

To apply special setups to individual atoms, identify them by their zero-indexed number in the atom list and use the full setup name. For example,

calc = Vasp(xc='PBE', setups={3: 'Ga_d'})

will treat the Ga atom in position 3 (i.e. the fourth atom) of the atoms object as special, with an additional 10 d-block valence electrons, while other Ga atoms use the default 3-electron setup and other elements use their own default setups. The positional index may be quoted as a string (e.g. {'3': 'Ga_d'}).

These approaches may be combined by using the ‘base’ key to access a default set, e.g.

calc = Vasp(xc='PBE', setups={'base': 'recommended', 'Li': '', 4: 'H.5'})

Spin-polarized calculation

If the atoms object has non-zero magnetic moments, a spin-polarized calculation will be performed by default.

Here follows an example how to calculate the total magnetic moment of a sodium chloride molecule.

from ase import Atoms, Atom
from ase.calculators.vasp import Vasp

a = [6.5, 6.5, 7.7]
d = 2.3608
NaCl = Atoms([Atom('Na', [0, 0, 0], magmom=1.928),
              Atom('Cl', [0, 0, d], magmom=0.75)],
             cell=a)

calc = Vasp(prec='Accurate',
            xc='PBE',
            lreal=False)
NaCl.set_calculator(calc)

print(NaCl.get_magnetic_moment())

In this example the initial magnetic moments are assigned to the atoms when defining the Atoms object. The calculator will detect that at least one of the atoms has a non-zero magnetic moment and a spin-polarized calculation will automatically be performed. The ASE generated INCAR file will look like:

INCAR created by Atomic Simulation Environment
 PREC = Accurate
 LREAL = .FALSE.
 ISPIN = 2
 MAGMOM = 1*1.9280 1*0.7500 

Note

It is also possible to manually tell the calculator to perform a spin-polarized calculation:

>>> calc.set(ispin=2)

This can be useful for continuation jobs, where the initial magnetic moment is read from the WAVECAR file.

Brillouin-zone sampling

Brillouin-zone sampling is controlled by the parameters kpts, gamma and reciprocal, and may also be set with the VASP parameters kspacing and kgamma.

Single-parameter schemes

A k-point mesh may be set using a single value in one of two ways:

Scalar kpts
If kpts is declared as a scalar (i.e. a float or an int), an appropriate KPOINTS file will be written. The value of kpts will be used to set a length cutoff for the Gamma-centered “Automatic” scheme provided by VASP. (See first example in VASP manual.)
KSPACING and KGAMMA
Alternatively, the k-point density can be set in the INCAR file with these flags as described in the VASP manual. If kspacing is set, the ASE calculator will not write out a KPOINTS file.

Three-parameter scheme

Brillouin-zone sampling can also be specified by defining a number of subdivisions for each reciprocal lattice vector.

This is the second “Automatic” scheme described in the VASP manual. In the ASE calculator, it is used by setting kpts to a sequence of three int values, e.g. [2, 2, 3]. If gamma` is set to ``True, the mesh will be centred at the \(\Gamma\)-point; otherwise, a regular Monkhorst-Pack grid is used, which may or may not include the \(\Gamma\)-point.

In VASP it is possible to define an automatic grid and shift the origin point. This function is not currently included in the ASE calculator. The same result can be achieved by using ase.dft.kpoints.monkhorst_pack() to generate an explicit list of k-points (see below) and simply adding a constant vector to the matrix. For example,

import ase.dft.kpoints
kpts = ase.dft.kpoints.monkhorst_pack([2, 2, 1]) + [0.25, 0.25, 0.5]

creates an acceptable kpts array with the values

array([[ 0. ,  0. ,  0.5],
       [ 0. ,  0.5,  0.5],
       [ 0.5,  0. ,  0.5],
       [ 0.5,  0.5,  0.5]])

However, this method will prevent VASP from using symmetry to reduce the number of calculated points.

Explicitly listing the k-points

If an n-by-3 or n-by-4 array is used for kpts, this is interpreted as a list of n explicit k-points and an appropriate KPOINTS file is generated. The fourth column, if provided, sets the sample weighting of each point. Otherwise, all points are weighted equally.

Usually in these cases it is desirable to set the reciprocal parameter to True, so that the k-point vectors are given relative to the reciprocal lattice. Otherwise, they are taken as being in Cartesian space.

Band structure paths

VASP provides a “line-mode” for the generation of band-structure paths. While this is not directly supported by ASE, relevant functionality exists in the ase.dft.kpoints module. For example:

import ase.build
from ase.dft.kpoints import bandpath

si = ase.build.bulk('Si')
kpts, x_coords, x_special_points = bandpath('GXL', si.cell, npoints=20)

returns an acceptable kpts array (for use with reciprocal=True) as well as plotting information.

Restart old calculation

To continue an old calculation which has been performed without the interface use the restart parameter when constructing the calculator

>>> calc = Vasp(restart=True)

Then the calculator will read atomic positions from the CONTCAR file, physical quantities from the OUTCAR file, k-points from the KPOINTS file and parameters from the INCAR file.

Note

Only Monkhorst-Pack and Gamma-centered k-point sampling are supported for restart at the moment. Some INCAR parameters may not be implemented for restart yet. Please report any problems to the ASE mailing list.

The restart parameter can be used , as the name suggest to continue a job from where a previous calculation finished. Furthermore, it can be used to extract data from an already performed calculation. For example, to get the total potential energy of the sodium chloride molecule in the previous section, without performing any additional calculations, in the directory of the previous calculation do:

>>> calc = Vasp(restart=True)
>>> atoms = calc.get_atoms()
>>> atoms.get_potential_energy()
-4.7386889999999999

New Calculator

A new VASP calculator is currently in BETA testing, see vasp2, which implements the calculator using the FileIOCalculator.