[Pw_forum] "Centering" Requirement of ESM

[Dr. Robert Molt Jr.]

Good evening,

I am confused by the "centering" requirements of ESM. It is stated that:

Requires cell with a_3 lattice vector along z,
      normal to the xy plane, with the slab centered
      around z=0. Also requires symmetry checking to be
      disabled along z, either by settingnosym 
<http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#nosym>  = .TRUE.
      or by very slight displacement (i.e., 5e-4 a.u.)
      of the slab along z.

I have constructed a slab which is symmetric about the xy plane and 
centered around z=0 contour. This is fine and good.

However, once I add the adsorbate molecule, I cannot have 
symmetry...unless it is intended that I add the same adsorbate molecule 
on the "bottom" of the simulation, too? I am unclear what is meant by 
the slab being "centered." Center of mass? Center of symmetry?


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