[Pw_forum] constraining Cartesian lattice vector components

[Nemeth, Karoly]

Paolo,

Thanks for your reply. I was not aware of the option "cell_dofree". However, the options there may not completely cover all possible situations. In general, I would like to constrain the upper triangle of the cell_parameters matrix to zero (perhaps as a default), and this does not seem available. In another case, when working with surface slabs, I would like to vary v1_x, v2x and v2y, while keeping v3z constrained to a nonzero value and all the rest of the elements constrained to zero.
In general, an additional control to what is available already in "cell_dofree" could be very useful by following the convention already used for constraining the Cartesian atomic positions, just adding 3 optional numbers (0-s or 1-s) at the end of the lines of the cell_parameter matrix. Anyway, I am a very frequent user of Quantum Espresso and I greatly appreciate the high quality of code you have developed.

Best regards:

Karoly
http://science.iit.edu/people/faculty/karoly-nemeth
 
________________________________________
From: Paolo Giannozzi [paolo.giannozzi at uniud.it]
Sent: Sunday, March 16, 2014 8:39 AM
To: PWSCF Forum
Cc: Nemeth, Karoly
Subject: Re: [Pw_forum] constraining Cartesian lattice vector components

Have you looked at option "cell_dofree"?

P.

On Sun, 2014-03-16 at 01:41 +0000, Nemeth, Karoly wrote:
> Dear Quantum Espresso developers and users,
>
> I was wondering whether it is possible to constrain individual Cartesian components of the lattice vectors during geometry optimization (vc-relax), at least for the ibrav=0 case. This would in principle be similar to constraining Cartesian atomic positions, however, it appears that this feature is not available in Quantum Espresso yet.
>
> Also, it appears that during vc-relax the cell is slightly rotated. I would expect that the first two lattice vectors remain along the x axis and in the xy plane while the third one may have non-zero x, y and z components. In other words, if I start the optimization with the upper triangle of the cell_parameters matrix being all zero, I would expect that it stays so, and only the diagonals and the lower triangle will vary, which includes 6 numbers only. These six non-zero numbers represent the 6 lattice parameters in Cartesian space. Allowing all 9 numbers to vary in the cell_parameters matrix introduces redundancy that may lead to the rotation of the cell and slows down convergence.
>
> I would greatly appreciate any advice in the above two (related) matters.
>
> Thanks:
>
> Karoly
> http://science.iit.edu/people/faculty/karoly-nemeth
>
>
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--
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222