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奋力拼搏

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[交流] 六角晶系结构参数的优化问题

由于初学VASP，不知道如何优化六角晶系结构参数，看侯老师讲义上的内容，还是不明白。真诚地希望各位高手们帮忙指教，谢谢！！

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1楼 2012-08-12 22:57:37

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奋力拼搏(金币+1): 谢谢参与
WDD880227: 金币+1, 多谢指教 2012-08-17 08:43:59
奋力拼搏: 金币+3, 非常感谢您的回复！！谢谢！1 2012-08-19 17:39:42

看你对精度有啥要求，如果要求不是太高的话，直接设置ISIF=3，弛豫就可，最后的结构就是优化后的结果

如果要求精度较高，就得做V-E曲线，vasp manual上的：
Accurate bulk relaxations with internal parameters (one)

The general message is: whenever possible avoid volume relaxation with the default energy cutoff. Either increase the basis set by setting ENCUT manually in the INCAR file, or use method two suggested below, which avoids doing volume relaxations at all. If volume relaxations are the only possible and feasible option please use the following step by step procedure (which minimizes errors to a minimum):

Relax from starting structure (ISMEAR should be 0 or 1, see section 6.38).
Start a second relaxation from previous CONTCAR file (re-relaxation).
As a final step perform one more energy calculation with the tetrahedron method switched on (i.e. ISMEAR=-5), to get very accurate and unambiguous energies (no relaxation for the final run). The final calculation should be done with PREC=High, to get very accurate energies.
A few things should be remarked here: Never take the energy obtained at the end of a relaxation run, if you allow for cell shape relaxations (the final basis set might not be isotropic). Instead perform one additional static run at the end.
The relaxation will give a structure which is correct to first order, the final error in the energy of step 3 is of second order (with respect to the structural errors). If you take the energy directly from the relaxation run, errors are usually significantly larger. Another important point is that the most accurate results for the relaxation will be obtained if the starting cell parameters are very close to the final cell parameters. If different runs yield different results, then the run which started from the configuration which was closest to the relaxed structure, is the most reliable one.

We strongly recommend to do any volume (and to lesser extend cell shape) relaxation with an increased basis set. ENCUT=1.3 default cutoff is reasonable accurate in most cases. PREC=High does also increase the energy cutoff by a factor 1.25. At an increased cutoff the Pulay stress correction are usually not required.

Accurate bulk relaxations with internal parameters (two)

It is possible to avoid volume relaxation in many cases: The method we have used quite often in the past, is to relax the structure (cell shape and internal parameters) for a set of fixed volumes (ISIF=4). The final equilibrium volume and the groundstate energy can be obtained by a fit to an equation of state. The reason why this method is better than volume relaxation is that the Pulay stress is almost isotropic, and thus adds only a constant value to the diagonal elements of the stress tensor. Therefore, the relaxation for a fixed volume will give an almost correct structure.

The outline for such a calculation is almost the same as in the previous section. But in this case, one has to do the calculations for a set of fixed volumes. At first sight this seems to be much more expensive than method number one (outlined in the previous section). But in many cases the additional costs are only small, because the internal parameters do not change very much from volume to volume.

Select one volume and relax from starting structure keeping the volume fixed (ISIF=4 see sec. 6.24; ISMEAR=0 or 1, see section 6.38).
Start a second relaxation from previous CONTCAR file (if the initial cell shape was reasonable this step can be skipped, if the cell shape is kept fixed, you never have run VASP twice).
As a final step perform one more energy calculation with the tetrahedron method switched on (ISMEAR=-5), to get very accurate unambiguous energies (no relaxation for the final run).
The method has also other advantages, for instance the bulk modulus is readily available. We have found in the past that this method can be used safely with the default cutoff. (see also section 9.2).