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