NEB forces units in OUTCAR

[emguilla]

Dear all,

For some reason, I wanted to have a look at the convergence of a cNEB calculation I already ran. EDIFFG was set to -1e-2 eV/A and once I "grep" the string "RMS" in OUTCAR, I found values that are below this criterion for all images. However, I wish I could look at individual atomic convergence of the forces, and when I look at the table below the line "POSITION TOTAL-FORCE (eV/Angst)" the maximum norm is way above the criterion. So I corrected with the table found below " NEB: Tangent" but I didn't help as I would expect.

Obviously, the atoms for which the forces are strong are not frozen. They are the ones that moves the most (CH3 on top of a surface).

I hope someone can help me with this issue.

Emerick Guillaume

[graeme]

It would help to see your OUTCAR or a .tar.gz of the calculation (without CHG* or WAV* files). But actually, I'm not sure what you are asking. Are you saying that the calculation converged when some forces were above the EDIFFG criterion?

[emguilla]

Dear Pr.,

Thank for your quick reply. If I look at the individual forces acting on atoms, yes, it looks like the calculation converged when forces were still way above the criterion. (However the RMS tag seems to confirm that it had converged).

EDIT: I wonder if the forces in below the "NEB: Tangent" are expressed in eV/Bohr ? Adding those forces to the forces below "TOTAL-FORCE" add up (once converted to eV/Angst) quite perfectly and gives a total result corresponding to RMS.

Best regards,

Emerick Guillaume

[graeme]

The band appears converged, but you have high forces in your 06 endpoint directory. You are certainly allowed to use a non-relaxed endpoint, but make sure that you know what you are doing. The forces on the endpoints are not considered in the NEB calculation.

If you are looking at a different image, let me know which one and I can help breakdown the forces.

Also, note that your reaction path is entirely downhill in energy, which is consistent with simple molecular adsorption. There is no indication of a barrier for this process.

[emguilla]

Indeed, I forgot to relax that particular endpoint (I'm well aware of that, it is a mistake on my part). However, the issue which I believe is now solved is more relevant to the units (eV/Angst or eV/Bohr) of the force coming from the pseudo-string in the OUTCAR. The so-called TOTAL-FORCE does not include the spring forces (which make sense obviously), but to check that the system has converged, one must account for the latter, but it seems they're written in eV/Bohr unlike the TOTAL-FORCE expressed in eV/Angst. Can you tell if I'm right about that ?

Thank you anyway !

Emerick

[graeme]

All forces are in eV/Ang; there are no Bohr units in the vasp code that I know of.

If you give me a specific force that you are looking at (which image, which iteration) I can help you sort this out. The TOTAL-FORCE does have the NEB projection and spring forces.

[emguilla]

There might be a confusion on my part ... Does the values below "NEB: Tangent" in OUTCAR gives the forces of the spring or only their direction for each atom ?

[graeme]

The NEB tangent is just a unit vector along the path at each image.

[emguilla]

There is still something I don't understand wrt to what is written below NEB: Tangent in the OUTCAR file. Take image 5 for instance. The 10 lines following "NEB: Tangent" for the latest iteration of image 5 are :

0.0048 -0.0043 0.0067
0.0037 -0.0071 0.0083
0.0022 0.0022 0.0015
0.0029 -0.0009 0.0029
-0.0009 0 0.0184
-0.5360 -0.0003 0.2519
0.0009 0 -0.0024
0.0037 0 0.0029
0.0048 0.0043 0.0067
0.0037 0.0071 0.0083
0.0022 -0.0022 0.0015
0.0029 0.0009 0.0029
0.0026 0 0.0029
0.0029 0 0.0033
0.0023 0 0.0018
0.0021 0 0.0023

What does those value mean ? They're not unit vectors, so I assume that they were forces, but it doesn't make sense with the force obtained below "TOTAL-FORCE" in the OUTCAR.

[graeme]

The NEB tangent is a single unit vector in the 3N dimensional space of the calculation, describing the direction along the minimum energy path.

[emguilla]

Thank you, I didn't understood those value as such. Using which value should I multiply it to get the actual forces coming from the springs ?

I'm very thankful for your support.