[Pw_forum] epitaxial thin film, dofree, and target pressure

[Han Hsu]

Dear PWscf developers and users,

I am currently studying an epitaxial thin-film system. I would like to find
the optimized the structure with the lattice parameters a and b being
constrained, namely, finding the optimized lattice parameter c (and the
internal structure).

I am aware that this procedure can be done manually. But that would cost
lots of CPU hours.

I am considering using cell_dofree, and I am wondering how this option
works. Suppose I put
cell_dofree= 'z' under the namelist &CELL. How should I specify the target
pressure?

For a thin film with constrained a, b and optimized c, the stress tensor
should look like

sigma_xx = sigma_yy = a non-vanishing value.
sigma_xy = sigma_yx = also a non-vanishing value
sigma_zz = sigma_xz = sigma_yz = 0

But when specifying press= 0, I think (maybe I am wrong) the program is
searching for a structure with

sigma_ij=0,
i,j=x,y,z.

How should I specify the target pressure (stress tensor) if I perform
vc-relax with dofree= 'z'?

Thank you in advance for your response and hint.

Han

-- 
*************************************************************************
Han Hsu, Ph.D.
Postdoctoral Associate
Department of Chemical Engineering & Materials Science
University of Minnesota
http://www.cems.umn.edu/~hsuhan <http://www.cems.umn.edu/%7Ehsuhan>
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