Copy all files from the tutor/diamond directory to a work directory, and proceed step by step:
cubic diamond comment line 3.7 universal scaling factor 0.5 0.5 0.0 first Bravais lattice vector 0.0 0.5 0.5 second Bravais lattice vector 0.5 0.0 0.5 third Bravais lattice vector 2 number of atoms per species direct direct or cart (only first letter is significant) 0.0 0.0 0.0 positions 0.25 0.25 0.25The positions can be given in direct (fractional) or Cartesian coordinates. In the second case, positions will be scaled by the universal scaling factor supplied in the second line. The lattice vectors are always scaled by the universal scaling factor.
4x4x4 Comment 0 0 = automatic generation of k-points Monkhorst M use Monkhorst Pack 4 4 4 grid 4x4x4 0 0 0 shift (usually 0 0 0)The first line is a comment. If the second line equals zero, k-points are generated automatically using the Monkhorst-Pack's technique (first character in third line equals ``M''). With the supplied KPOINTS file a Monkhorst-Pack grid is used for the calculation.
> vaspAgain this command will work properly only, if the vasp excecutable is located somewhere in the search path. The search path is usually supplied in the PATH variable of your UNIX shell. For more details, the user is refered to a UNIX manual.
After starting VASP, you will get a output similar to
VASP.4.4.3 10Jun99 POSCAR found : 1 types and 2 ions LDA part: xc-table for CA standard interpolation file io ok, starting setup WARNING: wrap around errors must be expected entering main loop N E dE d eps ncg rms rms(c) CG : 1 0.1209934E+02 0.120E+02 -0.175E+03 165 0.475E+02 CG : 2 -0.1644093E+02 -0.285E+02 -0.661E+01 181 0.741E+01 CG : 3 -0.2047323E+02 -0.403E+01 -0.192E+00 173 0.992E+00 0.416E+00 CG : 4 -0.2002923E+02 0.444E+00 -0.915E-01 175 0.854E+00 0.601E-01 CG : 5 -0.2002815E+02 0.107E-02 -0.268E-03 178 0.475E-01 0.955E-02 CG : 6 -0.2002815E+02 0.116E-05 -0.307E-05 119 0.728E-02 1 F= -.20028156E+02 E0= -.20028156E+02 d E =0.000000E+00 writing wavefunctionsVASP uses a self-consistency cycle with a Pulay mixer and an iterative matrix diagonalisation scheme to calculate the Kohn Sham (KS) ground-state. Each line corresponds to one electronic step, and in each step the wavefunctions are iteratively improved a little bit, and the charge density is refined once. A copy of stdout (that's what you see on the screen) is also written to the file OSZICAR.
The columns have the following meaning: Column N is counter for the the electronic iteration step, E is the current free energy, dE the change of the free energy between two steps, and d eps the change of the band-structure energy. The column ncg indicates how often the Hamilton operator is applied to the wavefunctions. The column rms gives the initial norm of the residual vector ( ) summed over all occupied bands, and is an indication how well the wavefunctions are converged. Finally the column rms(c) indicates the difference between the input and output charge density. During the first five steps, the density and the potentials are not updated to pre-converge the wavefunctions (therefore rms(c) is not shown). After the first five iterations, the update of the charge density starts. For the diamond example, only three updates are required to obtain a sufficiently accurate ground-state. The final line shows the free electronic energy F after convergence has been reached.
More information (for instance the forces and the stress tensor) can be found in the OUTCAR file. Please check this file in order to get an impression which information can be found on the OUTCAR file.
Another important file is the WAVECAR file which stores the final wave functions. To speed up calculations, VASP usually tries to read this file upon startup. At the end of calculations, the file is written (or if it exists overwritten).
NSW = 10 ! allow 10 steps ISIF = 2 ! relax ions only IBRION = 2 ! use CG algorithmto the INCAR file. (At this point you might find it helpful to read section 6.22).
In order to find the minimum, VASP performs a line minimisations of the energy along the direction of the forces (see 6.22). The line minimisation, requires VASP to take a "small" trial step into the direction of the force, then the total energy is re-evaluated. From the energy change and the initial and final forces, VASP calculates the position of the minimum. For carbon, the automatically chosen trial step is much too large, and VASP can run more efficiently, if the parameter POTIM is set in the INCAR file:
POTIM = 0.1 ! reduce trial stepDo that and start once again from a more exited structure (i.e. 0.20,0.20,0.20).
At the end of any job, VASP writes the final positions to the file CONTCAR. This file has the same format as the POSCAR file, and it is possible to continue a run, by copying CONTCAR to POSCAR and running VASP again.
ISIF = 3 ! relax ions + volume POTIM = 0.1 ! you need to specify POTIM as welland start once again. If ISIF is set to 3, VASP relaxes the ionic positions and the cell volume.
Do not forget to check the OUTCAR file from time to time.
EDIFF = 1E-7 ! very high accuracy required 10^-7 eVin the INCAR file. Start from the CONTCAR file of the last calculation (i.e. copy CONTCAR to POSCAR).
Then use the default cutoff. The stress is now -43 kBar. This yields an estimation of the possible errors caused by the basis set incompleteness. (You might correct the relaxation by setting
PSTRESS = -43 ! Pulay stress = -43 kBin the INCAR file, but it is usually preferable to increase ENCUT).