[Pw_forum] Variable-cell optimization with constraints

[Paolo Giannozzi]

On Saturday 07 January 2006 21:50, Natalie Holzwarth wrote:

> My question has to do with the use of the Parrinello-Rahman dynamics
> to optimize a cell while keeping the symmetry.   In particular, we have
> hexagonal symmetry (ibrav=4) which apparently requires the lattice 
> vectors to have the form (a,0,0), (-a/2, a*sqrt(3)/2,0) (0,0,c).   On the
> first lattice move, this symmetry is broken and the program stops with
> the error message
>               from checkallsym : error #         2 
>               not orthogonal operation

from the users'guide:
--
Variable-cell optimization may occasionally break the starting
symmetry of the cell. When this happen, the run is stopped
because the number of k-points calculated for the starting
configuration may no longer be suitable. Possible solutions:
- start with a nonsymmetric cell
- use a symmetry-conserving algorithm: the Wentzcovitch
algorithm ("cell_dynamics='damp-w'") shouldn't break the symmetry.
--

Note that the current 'stable' version has a bug that causes this same 
'non orthogonal operation' error when restarting from a previous run 
in a variable-cell optimization. This should be fixed in the CVS version.

Paolo

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy