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 (imageimage, imageend point) are the same number, after linear interpolation.
Thanks a lot !.
Best regards,
NEB distances in the MEP
Moderator: moderators

 Site Admin
 Posts: 1899
 Joined: Tue Apr 26, 2005 4:25 am
 Location: University of Texas at Austin
 Contact:
Re: NEB distances in the MEP
Yes, that is all correct. The precision to which the equaldistance 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 equaldistance criterion is maintain on either side of the climbing image, but not across the entire band.
If you use the climbing image, the equaldistance criterion is maintain on either side of the climbing image, but not across the entire band.
Re: NEB distances in the MEP
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
Any help would be appreciated,
var

 Site Admin
 Posts: 1899
 Joined: Tue Apr 26, 2005 4:25 am
 Location: University of Texas at Austin
 Contact:
Re: NEB distances in the MEP
You should not have to change the spring constant (ever).
Are you using the climbing image? If so, this behavior is expected.
Are you using the climbing image? If so, this behavior is expected.
Re: NEB distances in the MEP
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 topfcc
NWRITE = 2
PREC = Accurate
LREAL = auto
ROPT = 2e4 2e4 2e4
ISTART = 0
ALGO = Fast
# IALGO = 48 ! 8: CG, 48: DIIS algorithm for electrons
SPRING = 5
IMAGES = 3
Electronic relaxation
NELM = 100
NELMDL = 7
EDIFF = 1e5
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: quasiNR, 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 (0001 and 0102) 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 (0304) 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.
System = water Rh111 diff topfcc
NWRITE = 2
PREC = Accurate
LREAL = auto
ROPT = 2e4 2e4 2e4
ISTART = 0
ALGO = Fast
# IALGO = 48 ! 8: CG, 48: DIIS algorithm for electrons
SPRING = 5
IMAGES = 3
Electronic relaxation
NELM = 100
NELMDL = 7
EDIFF = 1e5
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: quasiNR, 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 (0001 and 0102) 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 (0304) 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.

 Site Admin
 Posts: 1899
 Joined: Tue Apr 26, 2005 4:25 am
 Location: University of Texas at Austin
 Contact:
Re: NEB distances in the MEP
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.
If you tar.gz that calculation, and make it available, I'll take a look.
Re: NEB distances in the MEP
The default is LCLIMB = True
You did not set LCLIMB = False, so these results are expected.
You did not set LCLIMB = False, so these results are expected.