NEB distances in the MEP

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var
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Joined: Fri Jun 22, 2012 11:59 am

NEB distances in the MEP

Post by var »

Hello,
in a NEB calculation the distances in the MEP between images must be the same (within 2 decimals ?). The distance in the MEP between any of the two end points and the nearest image should be the same as between images after optimization ? Initially, all the distances between nearest neighbors (image-image, image-end point) are the same number, after linear interpolation.
Thanks a lot !.
Best regards,
graeme
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Re: NEB distances in the MEP

Post by graeme »

Yes, that is all correct. The precision to which the equal-distance restraint is achieved is determined by the spring constant between images and the force criterion which defines convergence.

If you use the climbing image, the equal-distance criterion is maintain on either side of the climbing image, but not across the entire band.
var
Posts: 3
Joined: Fri Jun 22, 2012 11:59 am

Re: NEB distances in the MEP

Post by var »

Many thanks, graeme ! ... I am calculating a diffusion with 3 images and SPRING = -5. After optimization the last of the 4 distances is shorter than the other (equal) 3. Should I change the SPRING ? to -4 or -6 ? I am trying both but I am not sure how to think 'weaker' or 'stronger' springs ?

Any help would be appreciated,
var
graeme
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Re: NEB distances in the MEP

Post by graeme »

You should not have to change the spring constant (ever).

Are you using the climbing image? If so, this behavior is expected.
var
Posts: 3
Joined: Fri Jun 22, 2012 11:59 am

Re: NEB distances in the MEP

Post by var »

Hello graeme (and forum), thank you for your answers. I am not doing cNEB. Please, give a look at the INCAR and at the "grep prev, 01/OUTCAR" and at the "grep prev, 03/OUTCAR" and tell me what is wrong in the INCAR or in my interpretation or something else. It is the diffusion of a H2O molecule on a metal surface.

System = water Rh111 diff top-fcc
NWRITE = 2
PREC = Accurate
LREAL = auto
ROPT = 2e-4 2e-4 2e-4
ISTART = 0
ALGO = Fast
# IALGO = 48 ! 8: CG, 48: DIIS algorithm for electrons
SPRING = -5
IMAGES = 3

Electronic relaxation
NELM = 100
NELMDL = -7
EDIFF = 1e-5
ISMEAR = 1 ! 0: Gaussian, electron smearing
SIGMA = 0.200
ENCUT = 400
%NBANDS = 60

Ionic relaxation
EDIFFG = -0.05 ! - means force is less than
NSW = 200 ! # of steps in optimization (default 0!)
IBRION = 2 ! 1: quasi-NR, 2:CG algorithm for ions
#NFREE = 10 ! number of DIIS vectors to save
POTIM = 0.25 ! reduce trial step in optimization
ISIF = 2 ! 0: relax ions, 1,2:relax ions,calc stresses, 3:relax ion+cell
ISYM = 0

#LWAVE = .FALSE. ! write WAVECAR?
#LCHARG = .FALSE. ! write CHGCAR?
#LVTOT = .FALSE. ! write LOCPOT?

Parallel
LPLANE = .TRUE.
NPAR = 2
NSIM = 2


grep prev, 01/OUTCAR

NEB: distance to prev, next image, angle between 0.736455 0.736455 179.999999
NEB: distance to prev, next image, angle between 0.740643 0.739667 176.180498
...
NEB: distance to prev, next image, angle between 0.922476 0.928102 128.978894
NEB: distance to prev, next image, angle between 0.923495 0.928974 129.011998

looks fine to me, both distances (00-01 and 01-02) in the MEP are the same number, but

grep prev, 03/OUTCAR

NEB: distance to prev, next image, angle between 0.736455 0.736455 180.000000
NEB: distance to prev, next image, angle between 0.734956 0.740863 175.884477
...
NEB: distance to prev, next image, angle between 0.931261 0.425674 171.843088
NEB: distance to prev, next image, angle between 0.932211 0.423212 171.953843

the last distance (03-04) is much smaller than the other three. Of course, you noticed that initially, the four distances are equal to 0.736455

Many thanks, whether you can help me or not ...
Regards.
graeme
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Re: NEB distances in the MEP

Post by graeme »

This does look fishy, but I can't tell what's going on without seeing the band.
If you tar.gz that calculation, and make it available, I'll take a look.
xph
Posts: 39
Joined: Tue Mar 13, 2012 9:33 pm

Re: NEB distances in the MEP

Post by xph »

The default is LCLIMB = True
You did not set LCLIMB = False, so these results are expected.
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