Messages

1

General Questions and Answers / Re: About stress tolerance in v11.2

« on: March 19, 2011, 23:12 »

Thanks! I used a grid_mesh_cutoff value of 75 Hartree in SGGA+U, which is probably not enough. Actually I did try LSDA+U, which converged faster and better. I am just not sure if LSDA+U is good for a transition metal like Ni?

Try LDA rather than GGA; sometimes it can be hard to get accurate gradients for GGA without a very large mesh cut-off, perhaps as high as 350 Ry. Geometries are not that sensitive to the exchange-correlation functional as energies are.

2

General Questions and Answers / About stress tolerance in v11.2

« on: March 18, 2011, 22:10 »

Hi,

I am using the latest version of ATK and found that the optimizeGeometry() method allows relaxation of a bulk material to extract its optimal lattice constant, according to the stress that is "felt" by the unit cell. A stress tolerance value is required (maximum_stress) for the method to determine when the relaxation stops. I am wondering if someone could give me suggestions on how to set this stress tolerance.

The default value given by the Script Generator is maximum_stress=0.05eV/Ang**3, which is approximately 8.01 GPa. This value seems orders of magnitude larger than the stress usually found in a metal. As an example, the yield strength of Ni is 14-35MPa, according to http://en.wikipedia.org/wiki/Yield_(engineering). When I tried to relax bulk Ni with a maximum_stress=1e-4eV/Ang**3 (16MPa), it took a long time and the lattice constant became 14.81Ang, while the experimental lattice constant is about 3.52Ang. 

I used SGGA+U for exchange-correlation, but set all the Hubbard U terms to zero.

Thanks!