[Pw_forum] Variable-cell optimization with constraints

[Natalie Holzwarth]

Dear Pw_forum,

     As a new PWscf user, I appreciate its many capabilities.   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
I am able to get past this error by making a slight change to vcsmd.f90,
restoring the correct symmetry to the lattice vectors after the new ones
have been calculated.    However, this seems like cheating and hopefully
there is a better method available.    Thanks in advance for your advice.

                                        Natalie Holzwarth

N. A. W. Holzwarth			email: natalie at wfu.edu
Department of Physics			www: http://www.wfu.edu/~natalie
Wake Forest University		        voice: 336-758-5510
Winston-Salem, NC 27109-7507            fax: 336-758-6142
U. S. A.