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1. Step: Bulk calculation

As a first step perform a bulk calculation, use the tetrahedron method and increase the number of k-points till your calculation is converged to the required accuracy. What is the required accuracy: Sorry, no general answer to this question, if you want to calculate surface energies within 10 meV you should probably increase the k-mesh till your energy is converged to 1 meV. Mind that a slab used to model the bulk usually contains around 20-100 atoms. This means that you need a very accurate bulk energy to get reliable surface energies. Once again, be as careful as possible. If you generate the param.inc file automatically chose "PREC=High" for the bulk calculation.

As a second step switch from the tetrahedron method to a finite temperature method. There are two possible choices at this moment:

• You can use the Gaussian method (ISMEAR=0) and a small SIGMA value (SIGMA=0.1). This method was previously used quite frequently, nevertheless we prefer the second choice:
• Use the method of Methfessel-Paxton with N=1. SIGMA should be as large as possible but the difference between the free energy and the total energy (i.e. the term " entropy T*S") in the OUTCAR should be negligible. The entropy term gives you a good estimation of the possible errors, and must be within the accuracy you want to obtain (taking the previous example approximately 1 meV).
From now on you should neither change ISMEAR nor SIGMA nor ENCUT. Repeat the bulk-calculations with an increasing set of k-points. The convergence speed with respect to the number of k-points should be almost the same as with the tetrahedron method.

Choose a reasonable set of k-points and the energy cut-off you want to use for the surface calculation and calculate the equilibrium lattice constant. Avoid wrap around errors ("PREC=High" for the makeparam utility). The lattice constant you obtain must be used as the lattice constant in the surface calculations. The free energy is the references value for all further calculations. Also calculate the entropy (i.e. the term

` entropy T*S`
in the OUTCAR file) or write down the total energy and the physical energy ( ).

Next: 2. Step: FFT-meshes and Up: 11.8 Surface calculations Previous: 11.8 Surface calculations

MASTER USER VASP
Mon Mar 29 10:38:29 MEST 1999