UT theoretical chemistry code forum

Hello everybody,

Ive just started working with Vasp, so I always have a lot of questions.

I would like to know, if the EDIFFG -Tag also effects the Image-atom, respectively in my case, the diffusing atom in a NEB-calculation.

My EDIFFG-Tag is set on -0,01 eV/A and my NEB-calculation reached the required accuracy., but when I check the TOTAL FORCES in the Image-OUTCARS, the forces (only) on the diffusing Atom are always higher than 0,01 eV/A .

But when I use the nebefs.pl-script, it says, that the Forces on each Image are lower than 0,01. Where can I find these forces from the nebefs-pl-script in the OUTCAR. What Forces are they?

I would really appreciate if anyone can help. Thank you.

Brendan

The issue here is that the relevant forces for an NEB calculation is different from that of a normal minimization. For an NEB calculation, the force that is minimized is the projection of the real force perpendicular to the reaction path, plus a spring force that keeps the images equally spaced. This is the force that needs to be less than you specified force criterion for convergence. This is also the force reported by the nebef.pl script.

Hello Graeme,

again, thank you very much for your reply.

So if I understand it correctly, the TOTAL-FORCE table only shows the true forces and somewhere in the OUTCAR is the Spring Force on the diffusing atom and the addition of these two ist what nebefs.pl gives me respectively what needs to be under the specified force criterion?

Does the script do the addition or is there a way to grep the information out of the OUTCAR?

Best wishes

Brendan

The TOTAL-FORCE is the NEB force.

Below that, there is some NEB projection information, such as:

NEB: Tangent
----------------------------------------------
0.00000 0.00000 -0.00000
.....

NEB: forces: par spring, perp REAL, dneb 0.005906 0.023649 0.000000
NEB: distance to prev, next image, angle between 1.164965 1.166147 164.528901
NEB: projections on to tangent (spring, REAL) 0.005906 -0.059084

Thank you again and sorry if I get a bit on your nerves, but I am a bit confused now.

Below, i have copied the TOTAL FORCES on the first 10 atoms of my supercell (Image 4) after the last relaxation. So these are the forces, when the NEB stopped and the required accuracy of 0,01 eV/A is reached.
But you can see, that the forces on Atom 9, - which is the diffusing atom - are much higher than 0,01 eV/A.

However the nebefs.pl-script tells me, that the force on Image 4 is 0,00975800. I can find this figur in the NEB-information - which I also copied below - under "FORCES: max atom:".

So why is the TOTAL-FORCE on atom 9 so different from that value? What is the realtion between the TOTAL-FORCES and that value and how is that calculated?

I hope you understand what my problem is and I would really appreciate it, if you try to answer my questions one last time in this matter.

Best wishes

Brendan


TOTAL-FORCE (eV/Angst)
-----------------------------------------------------------------------------------
-0.002577 -0.001999 -0.002186
-0.000178 -0.001650 -0.001837
0.003903 0.007611 -0.000756
0.001575 0.002410 -0.000863
-0.006993 0.000921 -0.005722
-0.002905 -0.000995 -0.001178
-0.000983 -0.000382 -0.000106
-0.001913 0.002760 -0.007924
0.648613 0.336471 0.283857
-0.001432 -0.000458 -0.000002



NEB: forces: par spring, perp REAL, dneb 0.005279 0.018696 0.000000
NEB: distance to prev, next image, angle between 0.652947 0.654003 162.854289
NEB: projections on to tangent (spring, REAL) 0.005279 0.827213
stress matrix after NEB project (eV)
0.00000 0.00000 0.00000
0.00000 0.00000 0.00000
0.00000 0.00000 0.00000
FORCES: max atom, RMS 0.009758 0.001184
FORCE total and by dimension 0.019427 0.008024
Stress total and by dimension 0.000000 0.000000

Check to see if the atoms with high forces are frozen. If so, those forces will be zeroed for the convergence test and the NEB force projections.

Hey Graeme,

none of the atoms are frozen. And atom 9 is the only atom of 270 atoms with such high forces. Do you know, where I can read how the Force projection is done exactly? Is that the projection perpendicular to the path for the real force respectively parrallel to the path for the spring Force?