NEB calc doesn't stop when convergence reached,

[olderandcolder]

Hi There,

I am performing a CI-NEB calculation. I have set

EDIFFG = 1E-05

but when the ionic convergence in energy goes below this value the calculation does not stop:

e.g:

1 F= -.44618348E+03 E0= -.44618348E+03 d E =-.446183E+03
2 F= -.44618353E+03 E0= -.44618353E+03 d E =-.468724E-04
3 F= -.44618356E+03 E0= -.44618356E+03 d E =-.286150E-04
4 F= -.44618357E+03 E0= -.44618357E+03 d E =-.164662E-04
5 F= -.44618359E+03 E0= -.44618359E+03 d E =-.211903E-04
6 F= -.44618362E+03 E0= -.44618362E+03 d E =-.267459E-04
7 F= -.44618369E+03 E0= -.44618369E+03 d E =-.718887E-04
8 F= -.44618382E+03 E0= -.44618382E+03 d E =-.129830E-03
9 F= -.44618397E+03 E0= -.44618397E+03 d E =-.145161E-03
10 F= -.44618409E+03 E0= -.44618409E+03 d E =-.116766E-03
11 F= -.44618412E+03 E0= -.44618412E+03 d E =-.333419E-04
12 F= -.44618417E+03 E0= -.44618417E+03 d E =-.470692E-04
13 F= -.44618436E+03 E0= -.44618436E+03 d E =-.196732E-03
14 F= -.44618454E+03 E0= -.44618454E+03 d E =-.179372E-03
15 F= -.44618460E+03 E0= -.44618460E+03 d E =-.568035E-04
16 F= -.44618468E+03 E0= -.44618468E+03 d E =-.804223E-04
17 F= -.44618472E+03 E0= -.44618472E+03 d E =-.416295E-04
18 F= -.44618474E+03 E0= -.44618474E+03 d E =-.178415E-04
19 F= -.44618476E+03 E0= -.44618476E+03 d E =-.184930E-04
20 F= -.44618477E+03 E0= -.44618477E+03 d E =-.131951E-04
21 F= -.44618485E+03 E0= -.44618485E+03 d E =-.833533E-04
22 F= -.44618492E+03 E0= -.44618492E+03 d E =-.648123E-04
23 F= -.44618496E+03 E0= -.44618496E+03 d E =-.410043E-04
24 F= -.44618501E+03 E0= -.44618501E+03 d E =-.479206E-04
25 F= -.44618499E+03 E0= -.44618499E+03 d E =0.170913E-04
26 F= -.44618499E+03 E0= -.44618499E+03 d E =-.378032E-05
27 F= -.44618500E+03 E0= -.44618500E+03 d E =-.522079E-05
28 F= -.44618504E+03 E0= -.44618504E+03 d E =-.431281E-04
29 F= -.44618508E+03 E0= -.44618508E+03 d E =-.349731E-04
30 F= -.44618514E+03 E0= -.44618514E+03 d E =-.599824E-04
31 F= -.44618522E+03 E0= -.44618522E+03 d E =-.810415E-04
32 F= -.44618529E+03 E0= -.44618529E+03 d E =-.672010E-04
33 F= -.44618537E+03 E0= -.44618537E+03 d E =-.849066E-04
34 F= -.44618545E+03 E0= -.44618545E+03 d E =-.832445E-04
35 F= -.44618544E+03 E0= -.44618544E+03 d E =0.135523E-04
36 F= -.44618545E+03 E0= -.44618545E+03 d E =-.852695E-05
37 F= -.44618545E+03 E0= -.44618545E+03 d E =-.600605E-06
38 F= -.44618545E+03 E0= -.44618545E+03 d E =-.133630E-06
39 F= -.44618545E+03 E0= -.44618545E+03 d E =-.284472E-05
40 F= -.44618544E+03 E0= -.44618544E+03 d E =0.117636E-04
41 F= -.44618544E+03 E0= -.44618544E+03 d E =-.430231E-05


Any suggestions on how to successfully terminate this calculation would be welcome.

Thanks!

[graeme]

Check to see if the convergence criterion is met for all of the images in the NEB calculation.

As an aside, it is much better to use a force-based convergence criterion (EDIFFG<0).

[olderandcolder]

Thank you for your reply.

As suggested I have switched to a force-based convergence criterion.

I have seen in another thread that you have said: "The true forces on the NEB images will not, in general, drop below EDIFFG. Remember, the images along a minimum energy path climb up over a barrier. It is the NEB forces that matter for convergence.", which very much makes sense and was the reason I had been using EDIFFG > 0.

However when I look at the forces it seems to me that the convergence criterion is based upon the true forces.

For example, in an NEB calc that successfully terminated with EDIFFG = -0.03 the forces on each image are:

1********:
NEB: forces: par spring, perp REAL, dneb 0.008044 0.041039 0.000000
2********:
NEB: forces: par spring, perp REAL, dneb 0.004864 0.054856 0.000000
3********:
NEB: forces: par spring, perp REAL, dneb 0.002813 0.040098 0.000000
4********:
NEB: forces: par spring, perp REAL, dneb 0.001306 0.048815 0.000000
5********:
NEB: forces: par spring, perp REAL, dneb 0.000559 0.026384 0.000000
6********:
NEB: forces: par spring, perp REAL, dneb 0.001107 0.046936 0.000000
7********:
NEB: forces: par spring, perp REAL, dneb 0.002724 0.045223 0.000000
8********:
NEB: forces: par spring, perp REAL, dneb 0.004766 0.066533 0.000000
9********:
NEB: forces: par spring, perp REAL, dneb 0.007901 0.078301 0.000000


I am guessing it is the perp REAL component I should be looking at for convergence here - which is higher than the convergence criterion.

while the true forces have dropped below the criterion.

1********:
FORCES: max atom, RMS 0.013222 0.005639
2********:
FORCES: max atom, RMS 0.021925 0.007426
3********:
FORCES: max atom, RMS 0.028241 0.005420
4********:
FORCES: max atom, RMS 0.021828 0.006585
5********:
FORCES: max atom, RMS 0.014334 0.003558
6********:
FORCES: max atom, RMS 0.021778 0.006331
7********:
FORCES: max atom, RMS 0.029203 0.006109
8********:
FORCES: max atom, RMS 0.026683 0.008994
9********:
FORCES: max atom, RMS 0.029560 0.010612


I wonder if you could tell me which forces I should be looking at for convergence? Also what you would suggest if the forces remain fairly high over the barrier ( ~ 0.1 ev/A)?

Many thanks for your time.

[graeme]

The NEB force is the real force perpendicular to the band (perp real) plus the spring forces parallel to the band (par spring). This NEB force is passed back to vasp and so when you are looking at FORCES: max atom, that is the NEB force. What does not converge, is the total real force which includes the component parallel to the band. If you think about a minimum energy path which has images climbing up a hill, the force down the hill will not get below your convergence criterion, but that is ok, because is the only the real force perpendicular to the path that matters for convergence (and the spring force to keep the images equally spaced).