[DFTB-Plus-User] Issue with the stress tensor calculation in DFTB+ v.1.1

[Goldman, Nir]

Hi All,

I've been testing the analytical stress tensor calculation in DFTB+ v.1.1, and I can't seem to get the analytical results to match my calculations of the numerical values. Attached as an example is a configuration of 32 H2O molecules at 1 g/cc equilibrated at 298 K for several ps. I’ve been running my tests by doing static geometry calculations (Driver = {}) with the SCC convergence set to 1E-12 in order to converge the energies as tightly as possible. I’ve been calculating two point numerical derivatives by changing the computational cell anywhere from +/-0.001 to +/-0.01 AA in the xx, yy, and zz directions.  For the attached configuration, the numerically computed stress in the xx directions appears to differ from the analytical result by almost two orders of magnitude, and is off by ~100% in the yy and zz directions.

Are there any known issues with the stress tensor calculation?  What systems were used to test the stress tensor calculations?  Perhaps I can run some
tests with the same input geometries in order to help figure things out.

Thanks,

Nir

--
Nir Goldman
Chemical Sciences Division, PLS
Lawrence Livermore National Laboratory
P.O. Box 808, L-282
Livermore, CA 94551-0808
Phone: (925) 422-3994
Fax: (925) 424-3281
https://www-pls.llnl.gov/?url=about_pls-scientific_staff-goldman_n
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