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: [email protected]
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