[Pw_forum] The issue confused me when calculating the elastic constants.

[Hongsheng Zhao]

Hi all,

You can find the following descriptions from this website: 
http://dft-blog.natanzon.eu/search/label/SIESTA,

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“SIESTA calculates stress tensor and you know stress/strain relationship 
from the textbooks. You have to relax your system properly to have your 
stress tensor be close to zero. Then manually deform lattice vectors in 
selected directions and obtain the value of stress. For example, if you 
want to calculate C11, you have to deform lattice vector a (I suppose it 
is (a,0,0)) to a(1+e), where e is your value of strain. Then you make 
the calculation with relaxing atomic positions but NOT lattice vectors 
(VariableCell false) and obtain the value of stress tensor Sigma_ij and 
find C11:

C11 = Sigma11/e”
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 From the above description, we can know that this method do a 
non-VariableCell calculation after manually deforming the lattice 
vector, in this case, changing a --> into a(1+e).  My issue is: due to 
the cell's shape been fixed, the cell's volume is also fixed.  But when 
we impose a strain along the direction of lattice vector a, I think the 
system should has a shrinking effect in the direction perpendicular to 
the strain's direction.  So I think, during the relaxing calculation, 
both the  atomic positions and the lattice vector perpendicular to 
lattice vector *a* should be changeable while keeping the   lattice 
vector *a* fixed after manually strained it.  Any hints on my confusion 
  will be highly appreciated.  Thanks in advance.

Best
-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China