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小木虫论坛-学术科研互动平台 » 计算模拟区 » 第一性原理 » Vasp&MedeA » 【讨论】讨论VASP里边ISYM参数的重要性！
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pingguo802929

木虫 (初入文坛)

[交流] 【讨论】讨论VASP里边ISYM参数的重要性！

各位虫友：
    据我的理解，对于VASP里边的ISYM参数，如果不对它进行设置，
让它保持默认值1或2，那么优化计算时，自始至终体系的初始
对称性将保持不变。 如果把它设置为0，则计算时将不考虑对称性，
优化出来的结构和初始结构的对称性有可能不一样。
    我是用PSTRESS参数给体系做加压的优化计算，我做了一些测试，感觉
ISYM对优化的结果（如能量，构型）影响并不是很大，大家对ISYM参数
有没有什么其他看法？加压优化计算时，需要把ISYM设置为0吗？

[ Last edited by aylayl08 on 2010-4-21 at 08:59 ]
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1楼2009-05-07 15:06:01
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nyoxd

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12楼2010-01-01 11:30:37
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y1ding

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wuchenwf(金币+1,VIP+0):十分感谢！ 5-7 22:49
虽然版上很多人认为"可以用PSTRESS参数给体系做加压的优化计算"

我可以认为:不可以用PSTRESS参数给体系做加压.PSTRESS的数值不是准确的.
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2楼2009-05-07 15:46:21
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y1ding

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wuchenwf(金币+2):十分感谢！
见VASP说明书
7.22 PSTRESS-tag
  

If the PSTRESS tag is specified vasp will add this stress to to stress tensor, and an energy



to the energy. This allows the user to converge to a specified external pressure. Before using this flag please read section 9.6.
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3楼2009-05-07 15:49:53
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y1ding

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spur(金币+2,VIP+0):辛苦！^_^ 5-8 13:38
9.6 Volume vs. energy, volume relaxations, Pulay Stress
  

If you are doing energy-volume calculations or cell shape and volume relaxations you must understand the Pulay stress, and related problems.

The Pulay stress arises from the fact that the plane wave basis set is not complete with respect to changes of the volume. Thus, unless absolute convergence with respect to the basis set has been achieved - the diagonal components of the stress tensor are incorrect. This error is often called ``Pulay stress''. The error is almost isotropic (i.e. the same for each diagonal component), and for a finite basis set it tends to decrease volume compared to fully converged calculations (or calculations with a constant energy cutoff).

The Pulay stress and related problems affect the behavior of VASP and any plane wave code in several ways: First it evidently affects the stress tensor calculated by VASP, i.e. the diagonal components of the stress tensor are incorrect, unless the energy cutoff is very large (ENMAX=1.3 *default is usually a save setting to obtain a reliable stress tensor). In addition it should be noted that all volume/cell shape relaxation algorithms implemented in VASP work with a constant basis set. In that way all energy changes are strictly consistent with the calculated stress tensor, and this in turn results in an underestimation of the equilibrium volume unless a large plane wave cutoff is used. Keeping the basis set constant during relaxations has also some strange effect on the basis set. Initially all G-vectors within a sphere are included in the basis. If the cell shape relaxation starts the direct and reciprocal lattice vectors change. This means that although the number of reciprocal G-vectors in the basis is kept fixed, the length of the G-vectors changes, changing indirectly the energy cutoff. Or to be more precise the shape of cutoff region becomes an elipsoide. Restarting VASP after a volume relaxation causes VASP to adopt a new ``spherical'' cutoff sphere and thus the energy changes discontinuously (see section 7.10).

One thing which is important to understand, is that problems due to the Pulay stress can often be neglected if only volume conserving relaxations are performed. This is because the Pulay stress is usually almost uniform and it therefore changes the diagonal elements of the stress tensor only by a certain constant amount (see below). In addition many calculations have shown that Pulay stress related problems can also be reduced by performing calculations at different volumes using the same energy cutoff for each calculation (this is what VASP does per default, see section 7.10), and fitting the final energies to an equation of state. This of course implies that the number of basis vectors is different at each volume. But calculations with many plane wave codes have shown that such calculations give very reliable results for the lattice constant and the bulk modulus and other elastic properties even at relatively small energy cutoffs. In a certain way constant energy cut-off are thus less prone to errors cause by the basis set incompleteness than constant basis set calculations. But it should be kept in mind that volume changes and cell shape changes must be rather large in order to obtain reliable results from this method, because in the limit of very small distortions the energy changes obtained with this method are equivalent with that obtained from the stress tensor and are therefore affected by the Pulay stress. Only large volume changes guarantee that the errors introduced by the basis set incompleteness are averaged out.
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4楼2009-05-07 15:50:09
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