The four mandatory les to start VASP calculations are the INCAR, POSCAR, POTCAR and KPOINTS-les.
INCAR
The INCAR-le is the control le for VASP calculations. Invoked by parameters, it contains information on all the options that are needed for the current calculations like, e.g. algorithms, exchange-correlation functionals and convergence as well as output options. The INCAR les I used for DFT-calculations are attached in the back of this chapter. The most important param-eters are:
The GGA-parameter species which GGA-functional should be used to calculate the exchange-correlation functional.
For metals, the step function used to evaluate the band-structure energy at 0 K, converges very slowly with the number of k-points included, as the occupancies at the Fermi-level jump discon-tinuously from 1 to 0. To circumvent this problem, partial occupancies are introduced and the step function is replaced by a smoother functions. ISMEAR oers the choice of this function.
For ISMEAR = −1, the step function is replaced by a Fermi-Dirac function whose width is de-scribed by SIGMA =kBT. The introduction of the smearing leads to a generalized free energy that needs to be minimized instead of the total energy, but the total energy can be extrapolated withσ →0. Errors in the forces introduced by this scheme are expected to be small.
The PREC ag determines the precision of the VASP calculations by, if not otherwise spec-ied, setting ENCUT and determining the size of the fast fourier transform grid for the calcu-lations.
ENCUT is the energy cut-o for the pseudopotentials. It includes plane-waves up to a kinetic energy ofEcut into the calculation and therefore determines the size of the basis set. It does not need to be specied, for there is already a default cut-o in the POTCAR-le (ENMAX).
The IBRION ag determines how the rst and second derivatives of the energy with respect to the ionic positions are calculated. This allows to perform MD-simulations and the relaxation of the ionic positions as well as the determination of phonon frequencies and Hessian matrix.
LCHARG: This ag determines whether or not the charge densities are written out during the calculations.
ISPIN determines if the calculations are to be done spin-restrictedly or unrestrictedly. If the H atom is above the surface, then, the spin needs to be considered in the calculations.
The parameter that determines which iterative matrix diagonalisation technique should be used to calculate the electronic ground state is ALGO. The best choices are the residual minimization method with direct inversion of iterative subspace (RMM-DIIS), the blocked Davidson algorithm or a mixture of both.
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fcc Au # Comment line to describe the file
4.0 # lattice constant
0.5 0.5 0.0 # cell-vector in x-direction 0.0 0.5 0.5 # cell-vector in y-direction 0.5 0.0 0.5 # cell-vector in z-direction 1 # number of atoms in the cell cartesian # coordinate system
0 0 0 # position of the atoms
Figure A.1.1.: The POSCAR le. Example for a1×1×1cell for a bulk-calculation.
POSCAR
The POSCAR-le contains the cell-geometry, the number of atoms and atomic species and their position for the problem under consideration. The rst line (see Fig. A.1.1) is reserved for a comment to give information about the calculation the POSCAR le is used in. The second line contains the lattice constant and the next three lines contain the cell matrix that contains the cell vectors inx- (line 3), y- (line 4) and z-direction (line 5). The lattice constant is always multiplied onto the cell matrix, so for cells in which the lattice constant is just a factor, it should be written in the second line. Due to the periodic boundary conditions VASP employs when one moves from calculating bulk-properties to surface properties one needs to separate the periodic images from one another in one direction to create a surface, usually done inz-direction. The amount of space by which the slabs are separated is called the vacuum distance.
Line 6 contains the number of atoms in the cell. If there is more than one species in the cell, the rst number denotes the number of atoms of the atomic species that comes rst in the POTCAR-le, followed by the number of atoms of the second species and so on. The following line (line 7) describes in which coordinate system the atomic positions will be given. The choice here is between Cartesian coordinates and direct coordinates. Direct coordinates give the atomic positions between 0.0 and 1.0 and can be obtained by multiplying the Cartesian coordinates with the inverse of the cell matrix. From line 8 on, the positions of all atoms are given. Here, the atomic species must follow in the same order as specied in line 6. This means if one writes a POSCAR-le containing one Au atom and two H atoms and writes in line 6 `1 2', then one has to write the position of the Au atom rst, followed by the positions of the two hydrogen atoms.
The POSCAR-le also determines the cell dimensions for a cell containing a surface. They are denoted bynx×ny×nz wherenx is the number of atoms inx-direction andny andnz the number of atoms in y- and z-direction, respectively. So, in the example in Fig. A.1.1, the cell dimensions are those of a 1×1×1 cell and a 2×2×4 cell would have four atoms in at its surface and four atoms inz-direction, that is four layers.
K-Points # comment line
0 # place to enter k-points manually Gammacentered #
26 26 1 # kpoint mesh in x, y, z-direction 0 0 0 # shift of kpoint mesh
Figure A.1.2.: The KPOINTS le. Example for a simulation with a surface.
POTCAR
The POTCAR-le contains the pseudopotentials for all the atomic species in the calculation.
It also provides information about the atoms, like their masses, their valences and specics with regards to the way the pseudopotential were created. Furthermore, they contain a default energy cut-o radius which is chosen such that, in a bulk calculation, the error in the cohesive energy would be less than 10 meV. If more than one atomic species appears in the calculations, the two corresponding POTCAR-les need to be agglutinated. Care has to be taken here that the positions given for the individual atomic species in the POSCAR-le correspond to their ordering in the POTCAR-le.
KPOINTS
The KPOINTS-le gives the mesh in which the Brillouin-zone is sampled. The k-point mesh can be given either directly (line 2, Fig. A.1.2) by giving specic coordinates or automatically by entering a mesh size (line 4). For the automatic mesh generation, the method that should be used for the generation needs to be entered in the KPOINTS-le in line 3. The last line in the KPOINTS-le allows to shift the automatically generated mesh.