How do I relax only the in-plane component of the unit cell?

Question

I know that &CONTROL='vc-relax' option calculation performs a unit cell relaxation, and that the ATOMIC_POSITIONS can be fixed as follows:

ATOMIC_POSITIONS crystal
  Fe    0.000 0.000  0.00  0  0  0
  Fe    0.500 0.500  0.00  0  0  0
  Se    0.500 0.000  1.33  0  0  1
  Se    0.000 0.500 -1.33  0  0  1

Which I believe tells me that only the z-component of the Se atoms within the unit cell are adjusted. (This I learned from a nice tutorial by RadeMaker [1]. The pw.x documentation itself is not clear about this.)

Can I do something similarly for the unit cell parameters themselves? For example...

CELL_PARAMETERS angstrom
  3.77 0.00 0.00  1  0  0
  0.00 3.77 0.00  0  1  0
  0.00 0.00 5.52  0  0  1

I can't find documentation detailing this, or anything clarifying on StackExchange or ResearchGate. I would very much like to prevent the boundary conditions from tilting out of the plane of the Fe atoms. Thanks!

[1] https://arxiv.org/abs/2011.09888

Answer 1

To play with parameters or vectors of unit cell, you can use following tag inside &CELL

cell_dofree CHARACTER
Default:    'all'
Select which of the cell parameters should be moved:
            
'all' :
 all axis and angles are moved
            
'ibrav' :
 all axis and angles are moved, but the lattice remains consistent
with the initial ibrav choice. You can use this option in combination
with any other one by specifying "ibrav+option". Please note that some
combinations do not make sense for some crystals and will guarantee that
the relax will never converge. E.g. 'ibrav+2Dxy' is not a problem for
hexagonal cells, but will never converge for cubic ones.
            
'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 v2_y are moved
            
'xz' :
 only v1_x and v3_z are moved
            
'yz' :
 only v2_y and v3_z are moved
            
'xyz' :
 only v1_x, v2_y, v3_z are moved
            
'shape' :
 all axis and angles, keeping the volume fixed
            
'volume' :
 the volume changes, keeping all angles fixed (i.e. only celldm(1) changes)
            
'2Dxy' :
 only x and y components are allowed to change
            
'2Dshape' :
 as above, keeping the area in xy plane fixed
            
'epitaxial_ab' :
 fix axis 1 and 2 while allowing axis 3 to move
            
'epitaxial_ac' :
 fix axis 1 and 3 while allowing axis 2 to move
            
'epitaxial_bc' :
 fix axis 2 and 3 while allowing axis 1 to move
Above tag may not work perfectly if cell is not orthogonal
            

If you don't want forces and stress to relax along certain direction, you have to edit subroutine init_dofree in file Modules/cell_base.f90 and recompile your QE.