Hi all,
I would like to ask if during an MD simulation we can fix one lattice constant. I am aware that we can use Conv ensemble, but the point here that I want to fix only the z direction while allows the system to relax in all the other dimensions. The ocell option works well for the Energy_minimaztion step . Can I apply ocell also for the MD simulation ?
Thank you!
Omar
Hi Omar,
I’m afraid you can’t use “ocell” for MD since it changes the optimisation variables and so this isn’t relevant in MD. You should be able to run with a fixed cell parameter, but you’ll need to use the flags to set this rather than “conv”. I’d also note that GULP is primarily a lattice dynamics code rather than MD and so if you are using a standard model (rather than one of the more specialist GULP models) then it would be more efficient to use LAMMPS etc for your calculation.
Regards,
Julian
Dear Julian,
I hope you are doing great.
First of all, thank you for your reply.
Indeed, the model I am using is better implanted in Gulp so moving to LAMMPS is not a good choice.
I tried to use the flags by adding 1 1 0 0 0 0 beside the lattice constant in the input file so that I can fix the Z axis but it is not working.
Is there is anyway that I can ask the MD to fix the Z direction while allow the x and y axis to relax.
again thank you!
Hi Omar
Unfortunately “not working” is not a very specific error message & so it’s hard for me to know what this means. 
If you can post your input file then it will be much easier to investigate.
Regards,
Julian
T_14.in.in (24.6 KB)
Dear Juilan,
I attached you the input file. In this file I did not mentioned conv or conp as a keyword because I am a little bit confused which keyword I should to use with the flags.
Thank you!
Hi Omar
Thanks for providing your input. On checking, I’ve realised that fixing cell parameters in a general way is not currently implemented, so sorry for raising your hopes. It’s possible to fix the angles or to keep the cell isotropic (since these things are commonly needed for liquids), but fixing select lengths needs to be added.
If you can only run with GULP due to the model, then one option to explore is to see if you can call GULP as a calculator from ASE to achieve what you want since they might have cell constraints as an option.
Regards,
Julian
Dear Julian,
Thank you !
Dear Julian,
Is there any plan to add this feature Soon?
Again, many thanks!
Hi Omar,
It’s not top of my list at the moment, but I’ll keep it in mind for when I have some free time.
Regards,
Julian