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,

## NEB distances in the MEP

**Moderator:** moderators

### Re: NEB distances in the MEP

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.

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.

### 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

### 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 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.

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.

### 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.