Question about the setting of DFT (Density Function Theory)-NEB (Nudged Elastic Band) calculation

I'm attempting to calculate Mg ion vacancy migration inside Pyrope unit cell. System has total 159 atoms, beginning and end geometry is optimized. For intermediate image, I'm using one coordinate that I acquired from crashed NEB case. This is not perfect intermediate geometry, but I thought this should be enough one to guide NEB.

1) Is there any rule of thumb to decide "num_of_images" based on moving distance of target atom? In my script, I wish to move one Mg atom from one position to next vacancy which is about 3.5 angstrom away. In this case, how many images for NEB would proper? For example, if I choose 1 image per 0.5 angstrom, then 8 images should be chosen including the beginning and the end. But I'm not sure if 1 image per 0.5 angstrom would be the proper value. How do you decide "number of images" per given distance?

2) How many core number should be chosen for NEB? I have some sense of core number decision based on my experience, but this is for relaxation. For NEB, I'm not sure how much core number is proper for given atom number or given number of images setting. I wish to learn if there are rule of thumb to decide core number for given NEB conditions.

3) How to decide intermediate image? First, I tried my best guess of Mg ion location between two vacancies as an intermediate image. Then, I'm using one from one of the crashed NEB simulation result. Is this good approach?

4) Can freezing atom helps convergence? (freezing = fixing x, y, z coordinate of atom) Is this better to freeze (put 1 1 1 beside the atom coordinate) the target atom for the beginning, intermediate, and end geometry? Or surrounding atoms around target atom? For now I'm not inducing any freeze to any atom in my NEB setting, but I'm curious if freezing helps convergence of NEB calculation.

Thank you

• It is completely unclear which program you're using. It is also unclear which level of theory you are aiming at. Nov 9, 2019 at 18:46
• @Martin-マーチン Hello Martin, it is quantum espresso, and I'm using PBEsol PP. Nov 10, 2019 at 20:25