# External calculators¶

Many external calculators can be used with ASE, including GPAW, Abinit, Vasp, Siesta, Quantum ESPRESSO, Asap, LAMMPS and many more, see Supported calculators for the full list.

## Setting up an external calculator with ASE¶

This tutorial will cover how to set up a basic calculation in ASE, using an external calculator. We will be using the vasp2 module in this example, other calculators can be used in a similar manner. For details please refer to the documentation of the specific calculators Supported calculators.

Important: ASE does not provide code or a license for VASP, and these must be aquired elsewhere. ASE only creates an interface with VASP, so that you can use the ASE provided tools together with VASP.

## Setting up¶

The first step, is to tell ASE how to execute VASP, and where to find the pseudopotentials. You will need to have two environment variables defined:

$export ASE_VASP_COMMAND="mpiexec$HOME/vasp/bin/vasp_std"
$export VASP_PP_PATH=$HOME/vasp/mypps


The first environment variable ASE_VASP_COMMAND is the default way to execute VASP, and should be defined in the same way, that you could normally execute a VASP run. Here we assume that the VASP executable, vasp_std, is located in $HOME/vasp/bin. Note, that if you want to execute VASP in parallel, this call should also include the MPI executable, which in this case is mpiexec. The second variable, VASP_PP_PATH, is the path to the VASP pseudopotentials. An additional (optional) variable for the vdw_kernel.bindat file, which is required when doing van der Waals calculations, where luse_vdw=True. $ export ASE_VASP_VDW=\$HOME/<path-to-vdw_kernel.bindat-folder>


Note, that this should target the folder, and not the file itself.

## Your first run¶

Now that ASE knows how to execute VASP, we can try setting up a simple calculation. First we set up an atoms object

from ase.build import molecule

atoms = molecule('N2')
atoms.center(vacuum=5)


To perform a VASP DFT calculation, we now set up a calculator object. Note, that we currently have a Vasp and Vasp2 object - the Vasp2 is a newer version of the calculator, and will eventually replace the original Vasp calculator. In this example, we will use the Vasp2 calculator.

from ase.calculators.vasp import Vasp2

calc = Vasp2(xc='pbe',  # Select exchange-correlation functional
encut=400, # Plane-wave cutoff
kpts=(1, 1, 1)) # k-points

atoms.calc = calc
en = atoms.get_potential_energy()  # This call will start the calculation
print('Potential energy: {:.2f} eV'.format(en))


Which results in the following output:

Potential energy: -16.59 eV


The flow of how ASE interfaces with VASP, is that ASE handles writing the input files, which are required for the run, and then executes the ASE_VASP_COMMAND, i.e. executes VASP. Once the VASP run is complete, ASE then reads all of the relevant files, in this case the OUTCAR, vasprun.xml and CONTCAR, and stores properties in the calculator object.

For more information on the capabilities of the VASP calculators, please refer to VASP and VASP 2.0. For other calculators, please refer to the Calculators page.