Cd Si volume relaxation: Difference between revisions

Latest revision as of 08:32, 14 November 2019

Overview > fcc Si > fcc Si DOS > fcc Si bandstructure > cd Si > cd Si volume relaxation > cd Si relaxation > beta-tin Si > fcc Ni > graphite TS binding energy > graphite MBD binding energy > graphite interlayer distance > List of tutorials

Task

Relaxation of the internal coordinates, volume and cell shape in cd Si.

Input

POSCAR

cubic diamond
   5.5
 0.0    0.5     0.5
 0.5    0.0     0.5
 0.5    0.5     0.0
  2
Direct
 -0.125 -0.125 -0.125
  0.125  0.125  0.125

INCAR

System = diamond Si
ISMEAR = 0; SIGMA = 0.1;
ENMAX  =  240
IBRION = 2; ISIF=3 ; NSW=15
EDIFF  = 0.1E-04
EDIFFG = -0.01

IBRION=2 conjugate-gradient algorithm.
ISIF=3 change of internal parameter, shape and volume simultaneously.

KPOINTS

k-points
 0
Monkhorst Pack
 11 11 11
 0  0  0

Calculation

To determine the equilibrium volume we can:
- Fit the energz over a certain volume range to an equation of state (see cd_Si).
- Alternatively we relax the structure with VASP "on the fly" (IBRION=2 and ISIF=3)

From equation of states we determine lattice parameter of $a=5.4687$ Å (volume scan plus Murnaghan EOS using ENMAX=400).

From relaxations using IBRION=2 and ISIF=3 we get $a=5.4684$ Å.

Difference can be due to pulay stress (especially when the relaxation starts far away from equilibrium):

-------------------------------------------------------------------------------------
Total       0.00155     0.00155     0.00155    -0.00000     -0.00000      0.00000
in kB       0.06056     0.06056     0.06056    -0.00000     -0.00000      0.00000
external pressure =        0.06 kB  Pullay stress =          0.00 kB
   
   
VOLUME and BASIS-vectors are now :
-----------------------------------------------------------------------------
 energy-cutoff :      400.00
 volume of cell :      40.88
     direct lattice vectors                 reciprocal lattice vectors
    0.000000000  2.734185321  2.734185321    -0.182869828  0.182869828  0.182869828
    2.734185321  0.000000000  2.734185321     0.182869828 -0.182869828  0.182869828
    2.734185321  2.734185321  0.000000000     0.182869828  0.182869828 -0.182869828

To remedy this increase the plane wave cutoff by at least 30% (here we used ENMAX=400 instead of 240) and use a small EDIFF.

Summary

Calculation of the equilibrium volume:
- FIt the energy over a certain volume range to an equation of state.
- When internal degrees of freedom exist (e.g. c/a), the structure must be optimized. Use a conjugate-gradient algorithm (IBRION=2) and at each volume do e.g. 10 ionic steps (NSW=10) and allow change of internal parameters and shape (ISIF=4).

Simpler but less reliable: relaxing all degrees of freedom including volume.
- To relax all degrees of freedom use ISIF=3 (internal coordinates, shape and volume).
- Mind pulay stress problem. Increase plane wave cutoff by 25-30% when the volume is allowed to change.

Download

diamondSivolrel.tgz

Overview > fcc Si > fcc Si DOS > fcc Si bandstructure > cd Si > cd Si volume relaxation > cd Si relaxation > beta-tin Si > fcc Ni > graphite TS binding energy > graphite MBD binding energy > graphite interlayer distance > List of tutorials

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@@ Line 1: / Line 1: @@
-{{Template:Bulk_systems}}
+{{Template:Bulk_systems - Tutorial}}
 == Task ==
@@ Line 39: / Line 39: @@
 *To determine the equilibrium volume we can:
-**Fit the energz over a certain volume range to an equation of state (see {{TAG|cd_Si}}.
+**Fit the energz over a certain volume range to an equation of state (see {{TAG|cd_Si}}).
 **Alternatively we relax the structure with VASP "on the fly" ({{TAG|IBRION}}=2 and {{TAG|ISIF}}=3)
-*From equation of states we determine lattice parameter of <math>a=5.4687</math> <math>\AA</math> (volume scan plus Murnaghan EOS using {{TAG|ENMAX}}=400).
+*From equation of states we determine lattice parameter of <math>a=5.4687</math> &Aring; (volume scan plus Murnaghan EOS using {{TAG|ENMAX}}=400).
-*From relaxations using {{TAG|IBRION}}=2 and {{TAG|ISIF}}=3 we get <math>a=5.4684</math> <math>\AA</math>.
+*From relaxations using {{TAG|IBRION}}=2 and {{TAG|ISIF}}=3 we get <math>a=5.4684</math> &Aring;.
 *Difference can be due to pulay stress (especially when the relaxation starts far away from equilibrium):
@@ Line 61: / Line 61: @@
 .734185321  0.000000000  2.734185321     0.182869828 -0.182869828  0.182869828
 .734185321  2.734185321  0.000000000     0.182869828  0.182869828 -0.182869828
+*To remedy this increase the plane wave cutoff by at least 30% (here we used {{TAG|ENMAX}}=400 instead of 240) and use a small {{TAG|EDIFF}}.
+=== Summary ===
+*Calculation of the equilibrium volume:
+**FIt the energy over a certain volume range to an equation of state.
+**When internal degrees of freedom exist (e.g. c/a), the structure must be optimized. Use a conjugate-gradient algorithm ({{TAG|IBRION}}=2) and at each volume do e.g. 10 ionic steps ({{TAG|NSW}}=10) and allow change of internal parameters and shape ({{TAG|ISIF}}=4).
+*Simpler but less reliable: relaxing all degrees of freedom including volume.
+**To relax all degrees of freedom use {{TAG|ISIF}}=3 (internal coordinates, shape and volume).
+**Mind pulay stress problem. Increase plane wave cutoff by 25-30% when the volume is allowed to change.
 == Download ==
-[http://www.vasp.at/vasp-workshop/examples/diamondSivolrel.tgz diamondSivolrel.tgz]
+[[Media:DiamondSivolrel.tgz| diamondSivolrel.tgz]]
-----
-[[VASP_example_calculations|To the list of examples]] or to the [[The_VASP_Manual|main page]]
+{{Template:Bulk_systems}}
+Back to the [[The_VASP_Manual|main page]].
 [[Category:Examples]]