UT theoretical chemistry code forum

Hi,I want to make a NEB calculation under high pressure,and there ara some questions;
1 IS there only one way to set the external pressure by seting lattice constants? whether the PSTRESS tag can be used or not?
2 Once we set a external pressure, does it mean that the tag ISIF can not be set to 3 to do any volume relaxation? There is abourt 10 percent error between energy barrier with and without volume relaxation at ambient pressure.
Thank you in advance!

This is a good question, and one that we don't know much about. You can set the external pressure, and ISIF=3 to have each image relax the cell volume. Images with different cell sizes, however, will appear to be separated by the distance moved by all atoms. This doesn't mean that the NEB will not converge, but there could be problems to watch for. Make sure that your frozen atoms (if any) do not move or have forces on them. This should be working properly, but if you see any (relative) motion of frozen atoms it means there is a bug to fix. Finally, there is a danger in having the cell relaxed at each image in that these degrees of freedom are not included in the reaction coordinate. As long as the cell dimensions change smoothly along the reaction path, you are probably safe, but if you see an abrupt change in the cell geometry, you can't trust the pathway. Good luck, this is new territory!