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Dear admin,
In the vasp wiki, I couldn't find ISIF tag which can be used to perform both position and volume optimization while keeping the cell shape fixed. After going through the source code, I updated 2 lines in 'src/main.F' file as following And now I can use ISIF=20 for position-volume optimization while keeping the cell shape fixed.
This worked for me, at least in simple geometry optimization (not molecular dynamics). I was wondering if this can be the right way to go.

Thanks

Dear ralf_tonner1,

Thank you for the information.
We are aware that there is no ISIF flag with the functionality you mention.
If a user desires to update positions and box volume ( without altering it's shape) it is recommended to use the ICONST file.
As described on this page
wiki/index.php/ICONST
under the header Restrictions on the volume and/or shape of the simulation cell
you can use the ICONST file to get the functionality you are looking for.

All the best Jonathan

Dear Jonathan,
Thank you very much. But if I get it right, the ICONST file is read only for molecular dynamics (IBRION=0). For simple geometry relaxation (IBRION=1 or 2) this will not work. or Am I missing something?

Thanks

Dear ralf_tonner1,

No you are completely right. I missed that you are looking for way
doing the optimization with BRION=1,2. But the feature you are looking for will be added in one
of the coming vasp releases.

Thank you for the information.
All the Best Jonathan

As a follow up: Adding in a setting for shape-controlled (with volume and atomic position DoF) would be hugely useful in my DFT work. I look forward to seeing this in the future!

@ jonathan_lahnsteiner2, in addition to the upcoming release for shape-controlled relaxations that you mentioned above, are there any plans to introduce a relaxation setting that would fix only the angles between the lattice vectors while running DFT? I think this would be greatly useful for studying metastable structures

Dear matthewkuner,

Yes I agree, this would be a very useful feature. The implementations of the
cell optimization was already done. Currently we are still testing the implementation.
In future versions it will be possible to keep the angles between the lattice vectors constant.
For the usage of the new feature, the vasp wiki will be updated as soon as the feature is available.

All the best Jonathan

Dear Admin,
I wanted to inquire about the implementation of geometry optimization with volume and position optimization using the fixed cell shape. I see in the source code that this feature has already been implemented in VASP 6.4.2 with ISIF=8 option. This information is not updated in the VASP wiki yet. Could you kindly confirm or provide any insights on whether this option is available for use?

Thanks

Dear ralf_tonner1,

Thank you for the information. As you state the feature is implemented and can be used.
The vasp wiki was updated accordingly.

All the best Jonathan

To admin:
Nice. Quick comment.
1. I believe the ISIF=8 feature is only valid for VASP>=6.4.2. If so, probably this information can be added to the wiki. I think, using vasp 5.4.4 and 6.3.x are still quite common.
2. Minor: "ISIF=0|1|2|3|4|5|6|7|8" <= At the top of the ISIF wiki page.

Thanks

Dear ralf_tonner,

Thank you for the reminder. The vasp wiki is updated accordingly.

Best Jonathan