Also pay attention to the k-point sampling. According to Figure 4 of Modelling Simul. Mater. Sci. Eng. 13 (2005) R1-R31
You *may* need to increase the k-point sampling substantially over what you would for the total energy. There test case is a metal (Tantalum) which are always more problematic than insulators. I also recommend this primer to anyone doing DFT, even the more experienced users on the list. It is very well written and points to many pitfals commonly encountered in DFT calculations. On 3/8/07, Yurko Natanzon <[EMAIL PROTECTED]> wrote:
Olexandr, Vasilii, thank you for suggestions. I'm trying to calculate bulk modulus of tetragonal, monoclinic and cubic zirconium dioxide. With deformation +-2% I get rather good results for tetragonal and monoclinic phase, but still not good results for cubic phase. I'll try your suggestion of fitting 5 points with deformation +- 1% . On 08/03/07, Vasilii Artyukhov <[EMAIL PROTECTED]> wrote: > In my experience, very good bulk moduli are produced by a simple quartic fit > to five points no far than 1% off the equilibrium, you don't actually need > lots of points or the Murnaghan fit (as long as you keep your points close > enough to the equilibrium value). Just run five calculations with fixed > lattice constants. Though you should check for the discontinuities caused by > a finite integration grid (MeshCutoff). It is also very important which > material you are studying, for example you shouldn't expect good results > with molecular crystals like fullerite. Could you please specify the > material? > > > 2007/3/1, Oleksandr Voznyy <[EMAIL PROTECTED]>: > > Do you have a cubic cell? > > Don't do VariableCell then, it destroys symmetry. > > (check the forces on atoms to be 0) > > If you know the exact symmetry of the cell (relative positions of atoms > > in the cell) then adjust only lattice constant. > > > > Then the effect of pressure in all direction will change the lattice > > equivalently as well. > > With the same relative coordinates just change the lattice constant in > > the range +-3% with the step 0.25-0.5% and check the total energies. > > > > -- Yurko Natanzon PhD Student Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences ul. Radzikowskiego 152, 31-342 Kraków, Poland Email: [EMAIL PROTECTED], [EMAIL PROTECTED]
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