zero diffusion/exhange barrier for an insolvable atom

[tiger-paw]

Hi folks,

If a foreign atom cannot dissolve into host lattice, can I still employ NEB to calculate on diffusion barrier?

I am calculating Y-Vac pair in a bcc lattice. The Y-Vac pair are originally sitting by 1/2[111] distance in the large bcc supercell before relaxation. To be more specific, in the starting image, before relaxation, the vacancy is originally at a corner site, 000, and the Y at the cubic center 0.5,0.5,0.5. After relaxation, the Y atom finally sits at about 2/3*1/2[111] distance from the corner 000. While in the ending image, before relaxation, the vacancy is at the cubic center and the atom at 000. After relaxation, the atom finally sits at about 1/3*1/2[111] distance from the corner 000. Therefore, the Y atoms are actually separated only by (1/3)*(SQRT(3)/2)*a = ~0.8 A in two end images. Consequently, for the following NEB calculation which I inserted evenly 4 intermediate images, I ended up with almost zero total energy variation among all images.

I have rechecked my calculations. I had a thought: this atom is a very large atom indeed. Furthermore it cannot really dissolve in Fe (the heat of solution is highly positive), so it can move a lot to vacancy in the bcc host during relaxation, making the atom-vacancy exchange barrier to be almost zero by NEB.

Any comments/suggestion regarding this? Many Thanks.

[graeme]

Yes, I saw your post on the vasp forum. The one thing that I would add is that you can use the climbing image to find a true saddle point along the pathway. If the climbing image converges, you will have the barrier, even it it's low. You can also minimize from each side of the saddle to make sure that it connects your two endpoints. In general, the NEB has no problem with particularly low or high barriers.

There is one additional check to make: see if your images are displaced from each other by a net translation. If you have no frozen atoms, it is possible for the images along the NEB to slide into the minima. The climbing image helps to avoid this potential problem. You can also freeze an atom away from your interstitial to help avoid translation. But anyways, see if the crystal is translating before worrying about how to solve this potential problem.