[QE-users] some queries about mono layer optimization and vacuum

[rekha sharma]

Dear Sir,

Good afternoon and happy new year to all of you.

I am new user here so I need a step by step reply.

Before sending this email I have read the posting rules.

I used Quantum Espresso 6.4.1 compiled with ifort on a 16 core CPU.

Without having any previous experience on any quantum simulation tool, I
spent almost six months to learn quantum espresso and for bulk system I
feel comfortable.

I have done some calculations with a binary semiconductor compound which
was vc-relxed for ecutwcf (65), ecurho (52) and k-mesh(9 9 3 0 0 0). The
convergence  criteria was less then 1meV for these parameters.
Pseudo-potentials are taken from PSlibrary (recommended there).

Now I want to do some calculation of its monolayer.
As I could not get any informative tutorials for this kind of calculations
so  I am asking here.
My doubts are on k-mesh, &CELL and vacuum for the mono-layer.

1. k-mesh:
Que. Should I keep the same k-mesh but reducing the k-point in
vacuum direction to 1?
i.e. 9 9 1 0 0 0 (9 9 3 0 0 0 was for bulk with 1meV convergence).
I have done a k-mesh convergence test and  no significant changes observed
after 9 9 1 0 0 0 k-grid.
My double arises here because materials cloud suggest 2 k-points in
vacuum direction.

2. &CELL
I have adopted two choices to do it.

A.

I have read the mailing list and found a link
<https://lists.quantum-espresso.org/pipermail/users/2017-October/039359.html>
where it is mentioned that how to do vc-relax of a mono-layer.
That user used
&CELL
cell_dofree='2Dshape'
/
to do a vc-relax.

With these flags (with k-mesh 9 9 1 0 0 0),
The relaxed data are below
Total force =     0.000008     Total SCF correction =     0.000000
          total   stress  (Ry/bohr**3)                   (kbar)     P=
-2.81                # Stress
     Final enthalpy =    -227.5797893621 Ry
Begin final coordinates
     new unit-cell volume =   1782.72164 a.u.^3 (   264.17209 Ang^3 )
     density =      0.85971 g/cm^3

CELL_PARAMETERS (alat=  1.00000000)
   6.675929970   0.000000000   0.000000000     # Please note a,b,c are same
what I have supplied in the input file.
   0.000000000   6.675929970   0.000000000
   0.000000000   0.000000000  40.000000000

B.

from pw.x input descriptions, I found "cell_dofree='2Dxy' " do to a
vc-relax in x-y plane.
So with below flags
&CELL
cell_dofree='2Dxy'
/

My results are
     Total force =     0.000013     Total SCF correction =     0.000002
               total   stress  (Ry/bohr**3)                   (kbar)     P=
  -0.05                # Stress is 0.5 kbar

     Final enthalpy =    -227.5803704614 Ry
Begin final coordinates
     new unit-cell volume =   1742.44890 a.u.^3 (   258.20428 Ang^3 )
     density =      0.87958 g/cm^3

CELL_PARAMETERS (alat=  1.00000000)
   6.600092607   0.000000000   0.000000000           # Please note here
that a and b has reduced from what I given into input file (6.675929970)
  -0.000000000   6.600092607   0.000000000
   0.000000000   0.000000000  40.000000000

My concern arises from the  stress and change in lattice parameters from
above two cases.
In the case-B  where I have adopted stress and Final enthalpy is more
stable then the case-A.

Que. Which approach I should use A or B?
Que. Is it okay if the cell parameters (a and b) deviates from the bulk
case?
Que. Or you can advice me how to do vc-relax for  a mono-layer.


C. For vacuum

Que. How to decide the vacuum size?
I have done a convergence test from 30 bohr to 50 bohr and the total energy
first decrease, reach to a lowest value of then starts to increase.

Below is the Total every for 30 Bohr to 50 Bohr vacuum size

-227.40963239 Ry   # 30
-227.43879270 Ry   # 31
-227.46529446 Ry   # 32
-227.48906558 Ry
-227.51005151 Ry
-227.52821961 Ry
-227.54356349 Ry
-227.55610674 Ry
-227.56589822 Ry
-227.57301424 Ry
-227.57755075 Ry
-227.57962022 Ry                  # with vacuum 41 bohr
-227.57934725 Ry
-227.57686484 Ry
-227.57231253 Ry
-227.56583286 Ry
-227.55757040 Ry
-227.54767074 Ry
-227.53627785 Ry
-227.52353424 Ry    # 49
-227.50957968 Ry    # 50

Any step wise reply will help me a lot.

Looking forward for a supportive response.

thank you very much sir,

Best wishes

Ms. Rekha
Ex-PG student,
LBS college, Jaipur
India
Mob.: +11 90-95 790 71 697
Email: rekha1997jpr at gmail.com
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