[CP2K-user] [CP2K:12574] What are the differences between running GEO_OPT and CELL_OPT?

[Patrick Gono]

Dear Konstantin,

A GEO_OPT calculation keeps the cell dimensions fixed and optimizes the
position of the atoms.
A CELL_OPT calculation optimizes the position of the atoms as well as the
cell parameters.

Versions of CP2K starting from 5.1 allow for the implementation of
constraints in a CELL_OPT calculation. You can run a CELL_OPT, and use the
CONSTRAINT keyword to fix the desired cell parameters. In the following
example, both the atom positions as well as the lattice parameter in the X
direction will be optimized:

&MOTION
    &CELL_OPT
        CONSTRAINT YZ
        ...
    &END CELL_OPT
    ...
&END MOTION

If you really want to only relax the cell parameters, you may always modify
the &CONSTRAINT section to fix the positions of all atoms:

&MOTION
    &CONSTRAINT
        &FIXED_ATOMS
            LIST 1..999              ! <----- your number of atoms here
        &END FIXED_ATOMS
    &END CONSTRAINT
    ...
&END MOTION

I believe mixing constraints and CELL_OPT is a nono in older versions of
CP2K (4.1 and older), you may need to investigate.

In general, however, I see no reason why not to use the former approach
where you simultaneously find the best lattice parameter along X and the
optimal atomic configuration in one go.

Yours sincerely,
Patrick Gono

On Thu, Nov 28, 2019 at 3:32 PM Konstantin Tamoev <
konstant... at gmail.com> wrote:

> Hello everyone,
>
> so, I have a molecular structure that I want to optimize and I understand
> the procedure involving GEO_OPT to do so. In my case I have to care for
> periodicities along let's say the x-axis. For that purpose, I use VMD to
> approximate a simulation cell for my structure as tight fitting as
> possible, so that between periodic images of the structure (along x-axis)
> there is not much overlap or distance.
> Now, I assumed that by running CELL_OPT in a subsequent step I would get a
> better simulation cell which I could use in another GEO_OPT step to get
> more exact results. But judging from the output of the CELL_OPT I'm
> guessing that CELL_OPT also changes xyz coordinates of my structure, i.e.
> tries to optimize the structure?
>
> Can someone explain to me the difference between the two (CELL- and
> GEO_OPT)? Maybe I just misunderstood what a cell optimization is.
>
> Regards,
>
> Konstantin
>
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