[DFTB-Plus-User] Forces in dftb+ (supercells and k-point mesh)

[Bálint Aradi]

Dear Anshu,

As with all method (DFTB, DFT, etc.), you can only trust forces, if your
k-point mesh is convergent. You can test this by increasing your k-point
sampling systematically and check how the forces change (as you have
already done). In some specific 2D materials (e.g. graphene) it may
require a lot of k-points to get forces converged at low electronic
temperatures.

Otherwise, please not that 1e-6 Hartree/Bohr is in my opinion a very
tight relaxation criterion, 1e-4 is usually fine enough for most purposes.

Best regards,

Bálint


On 16.03.20 08:45, Anshu Gaur via DFTB-Plus-User wrote:
> 
> Dear all,
> 
> I am trying to see effect of forces (of different origins such as
> coulombic, van-der walls, strain etc.) on the properties of a structure.
> I have relaxed the structure (< 1e-6 Hartree/Bohr) using a sufficiently
> large k-point mesh. However, if I change the k-point mesh size (smaller)
> or make a supercell of the original structure, I see large forces in the
> structure (without changing atomic positions or lattice parameters).
> Also, I notice that the x-component of forces on atoms is the most
> affected (while y- and z-components remain unaffected). I have checked
> this by changing k-mesh in x-, y- and z- and also making supercells in
> x- and z- directions, the x-component of the forces always shows large
> values.
> Can the calculation of forces at any other k-point mesh be trusted, if
> the structure is relaxed using a different k-point mesh? or we should
> always use the same k-point mesh. Also, can we make a supercell and
> trust the force calculations?
> 
> My structure is 2D with vacuum in y- and extended in x- direction
> (ribbon like structure, continuous in x- and z-).
> 
> Regards,
> Anshu
> 
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-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-aradi/


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