Cd Si volume relaxation: Difference between revisions
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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) | ||
Revision as of 20:24, 23 June 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 (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
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