VASP

This tutorial provides an introduction to the basics of VASP and its usage. We will guide you through running your first VASP calculation, using an FCC silicon crystal as an example.

Before running calculations with VASP, several essential files must be present in your working directory. These files serve distinct purposes, and all are required for VASP to function properly. The four key files are:

INCAR POTCAR KPOINTS POTCAR 

INCAR

The INCAR file is the primary input file for VASP, controlling the tasks to be performed and their execution methods. The file includes various tags that specify algorithms and define the parameters VASP will apply during calculations. If certain parameters are not explicitly set, VASP automatically uses default values that can be found on the vasp wiki page.

For the Silicon crystal structure, we will use this INCAR:

System = fcc Si
ISTART = 0 ; ICHARG = 2
EDIFF = 1e−4
EDIFFG = 1e-3 #EDIFF×10
ENCUT = 240
ISMEAR = 0 ; SIGMA = 0.1
NPAR = 4
NSW = 0

Some important flags we use include EDIFF, which specifies the convergence criterion for the electronic density, signaling VASP to stop iterating once this condition is met. Similarly, EDIFFG refers to the convergence criterion for ionic relaxation. Another notable flag is NSW, which defines the maximum number of ionic steps VASP will attempt to achieve convergence if the EDIFFG condition is not satisfied.

There are around 300 flags that can be implemented in the INCAR file. To understand the purpose of each, refer to the VASP wiki.

POSCAR

The POSCAR file is a critical input for VASP, formatted as a plain text file that defines the lattice geometry and ionic positions. Creating a POSCAR file is typically the first step in VASP-based research. It can be manually written or obtained from online resources and crystallographic databases, with the Materials Project being a commonly used source for downloading POSCAR files. It is worth to mention that in our group, POSCAR files are often sourced from the Materials Project database

For the Silicon crystal structure, we will use this POSCAR:

fcc Si:
 3.9
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
   1
cartesian
0 0 0

In this POSCAR, we observe that the lattice parameter is 3.9 Å, the unit cell vectors are represented in their reduced form, corresponding to the primitive unit cell. There is only one atom in the unit cell, so there is a single line for the atomic coordinates.

KPOINT

The KPOINTS file specifies the Bloch vectors (k points) used for sampling the Brillouin zone, a crucial step in achieving convergence for electronic minimization calculations. A regular mesh is the most common method for selecting k points, with the number of points in each direction chosen approximately inversely proportional to the corresponding unit cell dimensions.

This is the KPOINT file for the example:

k-points
 0
Monkhorst Pack
 11 11 11
 0  0  0

The KPOINTS file structure:

1. First Line: Comment describing the k-point scheme.
2. Second Line: It asks if you want automatically generated k-points, and the answer is no (0).
3. Third Line: Specify mesh type (G/g for Γ-centered, M/m for Monkhorst-Pack).
4. Fourth Line: Define subdivisions 𝑁 1 , 𝑁 2 , 𝑁 3 along reciprocal lattice vectors.
5. Fifth Line (Optional): Shift the mesh by (𝑠 1 , 𝑠 2 , 𝑠 3 ).

POTCAR

The POTCAR file is a mandatory input file that contains information necessary for calculating the approximation of the exchange-correlation potential and pseudopotentials for each atomic species. All pseudopotentials for the elements must be concatenated into a single POTCAR file, with the elements listed in the same order as they appear in the POSCAR file. It is essential that the order of the elements in the POTCAR file exactly matches the order defined in the POSCAR file.

In our case, we use the PAW method for pseudopotentials and the PBE method to approximate the exchange-correlation potential. The VASP POTCAR files are from version 64 and are located on the cluster at:

/bmd/shared/vasp/recommended-potpawPBE64

Congratulations, you have learned how to run VASP. Now you can learn how to use VASP on the POWER cluster in Tutorial 9