Vasp uses three input files to make any calculation, INCAR, POSCAR, POTCAR and KPOINTS. You must have these three files in the same directory in order that VASP can make a run.
INCAR
INCAR file have the main instructions of the calculation, wether you use Molecular Dynamics or DFT, or if you want to make a geometry relaxation or a single point. It also gives special directives like magnetic properties, or the convergence criteria. The next command lines are the main instruction we will use during this tutorial.
# General input for VASP #=========================================== # General characteristics System = Graphene # Calculation Title PREC = NORMAL # Options: Normal, Medium, High, Low ENCUT = 400 # Kinetic Energy Cutoff in eV ISTART = 0 # Job: 0-new 1-cont 2-samecut ICHARG = 2 # initial charge density: 1-file 2-atom 10-cons 11-DOS ISPIN = 1 # Spin Polarize: 1-No 2-Yes # Electronic Relaxation (SCF) NELM = 60 # Max Number of Elec Self Cons Steps NELMIN = 2 # Min Number of ESC steps NELMDL = 10 # Number of non-SC at the beginning EDIFF = 1.0E-05 # Stopping criteria for ESC LREAL = .TRUE. # Real space projection IALGO = 48 # Electronic algorithm minimization VOSKOWN = 1 # 1- uses VWN exact correlation ADDGRID = .TRUE. # Improve the grid accuracy # Ionic Relaxation EDIFFG = 1.0E-04 # Stopping criteria for ionic self cons steps NSW = 0 # Max Number of ISC steps: 0- Single Point IBRION = 2 # Ionic Relaxation Method: 0-MD 1-qNewton-RaphsonElectronic 2-CG ISIF = 1 # Stress and Relaxation: 2-Ion ADDGRID = .TRUE. # Improve the grid accuracy SIGMA = 0.10 # Insulators/semiconductors=0.1 metals=0.05 ISMEAR = 0 # Partial Occupancies for each Orbital # -5 DOS, -2 from file, -1 Fermi Smear, 0 Gaussian Smear # Parallelization NPAR=8 NCORE=8
The most important tags are:
-For large cells you might loose the stopping criteria at the first steps, then you can tighten it.
-For single point calculation you have to set NSW=0, vasp will only make one ESC cycle.
-For molecule+surface relaxations you have to tight the ion force criteria as well as to set NSW=20, to see how system works.
-For CHG genereration in a DOS calculation you have to set a large NELM, like NELM=100-150. For a good charge file.
POSCAR
This file specify the box size and the ion possitions in it.
The first line is treated like a comment, where you usually put a filename.
The second one is a parameter whichs is the nth times the cell vectors (the box).
Third one has the three real unit vectors of the cell.
The next seccion specifies the ion possitions, it might be on cartessian coordinates or direct coordinates, the second type are the ion possitions in percentage of the unit vector.
For examples if a ion is at 0.5 0.5 0.5 it means that is in the center of the box.
Example system# Title or comment 1 # 1 times the unit cell 2.4500 0.0000 0.0000 # unit vector a 0.0000 2.4500 0.0000 # unit vector b 0.0000 0.0000 2.4500 # unit vector c C O # Type of atoms in your unit cell 1 2 # Number of atoms, here 1 Carbon and 2 Oxygen atoms Direct # Type of coordinates, in this case related to the cell 0.0000 0.0000 0.0000 # Carbon is at the origin of the cell 0.2500 0.0000 0.0000 # First Ox is at 0.25 times a unit vector 0.0000 0.1500 0.0000 # Second Ox is at 0.15 times b unit vector
KPOINTS
This file specify the number of K points in the K mesh to make the integrals over the Brillouin Zone.
Vasp can either generate the K-mesh with several methods or to read the K-mesh from a file.
Automatic Mesh # Generates Automatically the K-mesh 0 Monkhorst-pack # Automatic Distribution of the mesh 5 5 5
POTCAR
This file contains all the basis set information to represent an atoms, the important data we need to have in mind are, the ENCUT which is the kinetic energy of each ion, and the RWIGS which is the Wigner-Seitz radius of the ion.
NOTE: if your calculation has more than one ion you have to concatenate the POTCAR files that represents each ion, preserving the order you use on the POSCAR file.
Here you can see a POTCAR file: POTCAR