Vasilii, thank you for advice, I'll check it. But this works only if I don't put any geometry constraints. If I want ie fix the angles, I never get even low values of stress components - only change of angles will allow that. So in this case I need to take "zero stress" into account. In some cases, a simple difference between deformed and undeformed stress components can improve results, but I need more accurate formula and for this I have to know the strain tensor which corresponds to stress tensor calculated by SIESTA.
On 27/03/07, Vasilii Artyukhov <[EMAIL PROTECTED]> wrote:
If your system is not too large, I'd advise that you determine the relaxed lattice constants from a series of fixed-cell calculations, especially if the system is more or less symmetrical. Although your stresses seem quite low to me, I think that the only way to improve them might be, indeed, to lower the stress tolerance. Note that since you are using a finite integration grid, no one can guarantee that you will ever get zero stresses. So, check for the integration grid cutoff as well, you might have to increase it at some point. 2007/3/27, Yurko Natanzon <[EMAIL PROTECTED]>: > Dear siesters, > I'm thinking on increasing the accuracy of calculating elastic > constants (i'm interested in shear ones but it doesn't matter). First > I relax a lattice to get a stress tensor zero, and then make > deformations to get a new stress tensor and calculate elastic > constants. But in fact the stress tensor of relaxed lattice is not > zero, it has a form like this one: > -0.000020 -0.000056 0.000233 > -0.000056 0.000053 -0.000033 > 0.000233 -0.000033 0.001350 > the values of this tensor in fact influence the results of elastic > constant calculations. In fact, if I get +1% deformation in one > direction and -1% - I may get totally different values of stress > tensor. Is there a way to take the influence of zero stress tensor > into account? > If I'll be able to know, which strain corresponds to this stress > tensor, I can find the lattice vectors of ideally relaxed lattice, and > this will solve the problem. > > I would be grateful for any advices regarding this. Of course, I may > try to set StressTolerance and Force Tolerance to some low values, but > this will take much more CG steps to fully relax the lattice and much > more time. Also in some cases I can't fully relax the lattice because > of constraints, but need to relax only several components of stress > tensor. > > -- > 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] >
-- 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]
