[Pw_forum] vc-relax

[Paolo Giannozzi]

On Mar 21, 2012, at 11:48 , tfcao at theory.issp.ac.cn wrote:

> Thank you very much, I will try in it again in the recent version.

>> You are using a rather old version of the code (4.1.1). I am
>> not sure if the implementation of constraints was correct
>> at that time

actually it does not work with the latest version as well, but at least
the documentation is correct:

         var cell_dofree -type CHARACTER {
             default { 'all' }
             info {
                Select which of the cell parameters should be moved:

                all     = all axis and angles are moved
                x       = only the x component of axis 1 (v1_x) is moved
                y       = only the y component of axis 2 (v2_y) is moved
                z       = only the z component of axis 3 (v3_z) is moved
                xy      = only v1_x and v_2y are moved
                xz      = only v1_x and v_3z are moved
                yz      = only v2_x and v_3z are moved
                xyz     = only v1_x, v2_x, v_3z are moved
                shape   = all axis and angles, keeping the volume fixed

                Beware: if axis are not orthogonal, some of the above  
options
                        will break symmetry

In fact, for hexagonal lattices, symmetry will be broken and the code  
will
stop after the first step. This behavior can be easily changed, though:
just edit file Modules/cell_base.f90, locate lines
             CASE ('xy')
               iforceh      = 0
               iforceh(1,1) = 1
               iforceh(2,1) = 1
add lines
               iforceh(1,2) = 1
               iforceh(2,1) = 1

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222