Cd Si volume relaxation: Difference between revisions
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== Task == | == Task == | ||
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*To determine the equilibrium volume we can: | *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) | **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> | *From equation of states we determine lattice parameter of <math>a=5.4687</math> Å (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> | *From relaxations using {{TAG|IBRION}}=2 and {{TAG|ISIF}}=3 we get <math>a=5.4684</math> Å. | ||
*Difference can be due to pulay stress (especially when the relaxation starts far away from equilibrium): | *Difference can be due to pulay stress (especially when the relaxation starts far away from equilibrium): | ||
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*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}}. | *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 == | == Download == | ||
[ | [[Media:DiamondSivolrel.tgz| diamondSivolrel.tgz]] | ||
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Back to the [[The_VASP_Manual|main page]]. | |||
[[Category:Examples]] | [[Category:Examples]] | ||
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:
- From equation of states we determine lattice parameter of Å (volume scan plus Murnaghan EOS using ENMAX=400).
- 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
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
Back to the main page.