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Hello, I am trying to calculate the migration energy of a He atom from a tetrahedral to an octahedral interstitial location in a Y2Ti2O7 system. I am using the Nudged Elastic Band method, but I am unable to get the system to converge.

Here is what my INCAR file contains:

SYSTEM = Y2Ti2O7
PREC = HIGH
IMAGES = 5
EDIFF = 1E-4
LSCALAPACK = .FALSE.
ISIF = 2
LCLIMB = .TRUE.
ICHAIN = 0
LTANGENTOLD = .FALSE.
SPRING = -5
ISTART = 0
ICHARG = 2
INIWAV = 1
ISPIN = 1
NSW = 400
IBRION = 3
SMASS = 2
POTIM = .1
ISMEAR = 0
SIGMA = .05
LREAL = .TRUE.
ALGO = FAST
LWAVE = .FALSE.
LCHARG = .FALSE.
NPAR = 32
NCORE = 5

Also, in calculating the initial and final positions, I have simply calculated the path for the intermediate images and my understanding is that the NEB method will determine the minimum energy path from there. However, I am concerned with some things in the way that I have done this. The cell geometry in each of the interstitial locations is different after full relaxation of a single He atom in each location (for initial and final locations). When calculating the path from the initial to the final location, does the change in the cell geometry need to be accounted for, or can I simply use the same cell geometry for all images? I know the fully relaxed configurations of both the initial and final positions, but keeping the cell shape and volume the same does not allow for the relaxed position of the final position to be in its fully relaxed configuration.

Any help is greatly appreciated.

In the NEB calculation you should keep the cell shape and volume fixed for all images.