Electrostatic corrections: Difference between revisions

From VASP Wiki
VisualWikitext
mNo edit summary
 
(12 intermediate revisions by 3 users not shown)
Line 1: Line 1:
For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size <math>L</math> of the supercell. Using methods discussed by Makov ''et al.''<ref name="Makov95"/> and Neugebauer ''et al.''<ref name="Neugebauer92"/>, VASP can correct for the leading errors (in many details, we have taken a more general approach, though).  
For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size <math>L</math> of the supercell. Using methods discussed by Makov ''et al.''<ref name="Makov95"/> and Neugebauer ''et al.''<ref name="Neugebauer92"/>, VASP can correct for the leading errors (in many details, we have taken a more general approach, though).  


== Summary of relevant INCAR tags ==
== Suggested combination of tags for electrostatic corrections ==
This section contains a summary of all the {{FILE|INCAR}} tags that are currently implemented for performing monopole, dipole and quadrupole corrections using VASP. Please see the relevant pages of the respective tags for more detailed information.
In cases where the system has no net charge and no net dipole moment, no specific tags need to be set and this section can be skipped.
=== Bulk ===
If the system has a net dipole or net charge, please follow the recommendations of [[Dipole_corrections_for_defects_in_solids|this]] wiki page.
=== Surfaces ===
If the system has a net dipole moment, a combination of {{TAG|IDIPOL}}=1,2,3 and {{TAG|LDIPOL}} tags may be used. The former corrects the energies, while the latter corrects the potential and forces. Optionally, {{TAG|DIPOL}} may be set. The following options may be used to improve convergence for this case.


{|cellpadding="5" cellspacing="0" border="1"
1. Use any of these tags only after pre-converging the orbitals without the {{TAG|LDIPOL}} tag
| Dimensionality of the system
| Does the system have net charge?
| Does the system have a net dipole moment?
| Relevant INCAR tags for monopole/dipole corrections
| Do energies converge with cell dimension?
|-
| 3D
| No
| No
| None
| Yes
|-
| 3D
| Yes
| No
|{{TAG|NELECT}}: to set the charge
{{TAG|LMONO}}: monopole corrections, only implemented for cubic cells


{{TAG|EPSILON}}: scales monopole correction by dielectric constant of the medium
2. The center of charge should be set in the {{FILE|INCAR}} file ({{TAG|DIPOL}}= center of mass)
| If the correct {{TAG|EPSILON}} value is used, the energies and relative energies will not depend on cell size. For solids, see {{TAG|LCALCEPS}} and {{TAG|LEPSILON}}
|-
| 2D
| No
| Yes
| {{TAG|IDIPOL}}=1,2,3: direction in which to apply the dipole correction to the total energy
{{TAG|LDIPOL}}: enable dipole correction to the potential and forces


{{TAG|DIPOL}}: center of mass to compute the dipole moment
3. Ensure that the cell is sufficiently large to determine the dipole moment with sufficient accuracy (see {{TAG|DIPOL}}). If the cell is too small, the charge might slash through the vacuum, causing very slow convergence. Often convergence improves with the size of the supercell.
| Energies do not depend on the vacuum used (if sufficient vacuum is available)
{{NB|warning|Surface calculations with a net charge result in total energies that do not converge. Relative energies may still be useful.}}
|-
| 2D
| Yes
| No
| {{TAG|NELECT}}: to set the charge
| Absolute energies do not converge with cell dimension, but energy differences might be useful
|-
| 2D
| Yes
| Yes
| {{TAG|NELECT}}: to set the charge
| Absolute energies do not converge with cell dimension, but energy differences might be useful
|-
| 0D (isolated molecules)
| Yes
| Yes
| {{TAG|NELECT}}: to set the charge
{{TAG|LMONO}}: monopole corrections, only implemented for cubic cells and only corrects the energy


{{TAG|LDIPOL}}: monopole corrections with corrections for the potentials
=== Wires ===
| Energies do not depend on cell dimension (for large enough cells)
Not implemented.
|-
| 0D (isolated molecules)
| Yes
| No
| {{TAG|NELECT}}: to set the charge
{{TAG|LMONO}}: monopole corrections, only implemented for cubic cells and only corrects the energy


{{TAG|LDIPOL}}: monopole corrections with corrections for the potentials
=== Molecules ===
| Energies do not depend on cell dimension (for large enough cells)
If the system has a net dipole moment, use the {{TAG|LDIPOL}} tag. The former corrects the energies, while the latter corrects the potential and forces. Optionally, {{TAG|DIPOL}} may be set.
|-
| 0D (isolated molecules)
| No
| Yes
| {{TAG|LDIPOL}}: dipole corrections with corrections for the potentials included
| Energies do not depend on cell dimension (for large enough cells)
|}
 
{{NB|tip| If an external electrostatic field is desired for slab, or molecular calculations, see {{TAG|EFIELD}}}}


== Current limitations ==
== Current limitations ==
Line 80: Line 27:
:Quadrupole corrections are only correct for cubic supercells (this means that the calculated 1/''L''<sup>3</sup> corrections are wrong for charged supercells if the supercell is non-cubic). In addition, we have found empirically that for charged systems with excess electrons ({{TAG|NELECT}}>{{TAG|NELECT}}<sub>neutral</sub>) more reliable results can be obtained if the energy after correction of the linear error (1/''L'') is plotted against 1/''L''<sup>3</sup> to extrapolate results manually for ''L''&rarr;&infin;. This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons.
:Quadrupole corrections are only correct for cubic supercells (this means that the calculated 1/''L''<sup>3</sup> corrections are wrong for charged supercells if the supercell is non-cubic). In addition, we have found empirically that for charged systems with excess electrons ({{TAG|NELECT}}>{{TAG|NELECT}}<sub>neutral</sub>) more reliable results can be obtained if the energy after correction of the linear error (1/''L'') is plotted against 1/''L''<sup>3</sup> to extrapolate results manually for ''L''&rarr;&infin;. This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons.
* Potential corrections are only possible for orthorhombic cells (at least the direction in which the potential is corrected must be orthogonal to the other two directions).
* Potential corrections are only possible for orthorhombic cells (at least the direction in which the potential is corrected must be orthogonal to the other two directions).
== Step-by-step instructions ==
=== Using the dipole correction for slab calculations ===
In this section, we discuss step-by-step instructions to use the dipole corrections for slab calculations.
'''Step 1:''' Create a system which has enough vacuum on either side of the surface normal. An example for such a structure is shown below, for an fcc-Aluminium with a carbon adsorbed on one of its surface terminations.
Al3C
1.0000000000000000
    2.8637824638055176    0.0000000000000000    0.0000000000000000
    1.4318912319027588    2.4801083645679673    0.0000000000000000
    0.0000000000000000    0.0000000000000000  20.0000000000000000
Al C
3 1
Direct
    0.8333333333333333    0.5000000000000000    0.3380865704891008
    0.1666666666666666    0.8333333333333334    0.4550000000000000
    0.4999999999999999    0.1666666666666667    0.5719134295108992
    0.4999999999999999    0.1666666666666667    0.6619134295108993
Note that the system has plenty of vacuum on either side. This empty space is important for the potential corrections needed for the {{TAG|LDIPOL}} tag.
'''Step 2:''' Switch on the dipole corrections to the energy, potential and forces. Optionally set the {{TAG|DIPOL}}
LDIPOL    = T
IDIPOL    = 3
DIPOL    = 0.5 0.5 0.5
'''Step 3 (Optional):''' View the dipole moment for the system using the following bash command,
grep dipolmoment OUTCAR | tail -1
In this example, we get the following output:
  dipolmoment          0.000000      0.000000      0.128389 electrons x Angstroem
which refers to the dipole moment along the three axes. Consistent with the {{FILE|POSCAR}} used in this example, only the last axis has a non-zero dipole moment.


== Related Tags and Sections ==
== Related Tags and Sections ==
Line 99: Line 82:


----
----
[[The_VASP_Manual|Contents]]


[[Category:Atoms and Molecules]][[Category:Monopole Dipole and Quadrupole Corrections]]
[[Category:Atoms and Molecules]][[Category:Monopole Dipole and Quadrupole Corrections]][[Category:Electrostatics]][[Category:Howto]]

Latest revision as of 12:10, 9 September 2024

For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size of the supercell. Using methods discussed by Makov et al.[1] and Neugebauer et al.[2], VASP can correct for the leading errors (in many details, we have taken a more general approach, though).

Suggested combination of tags for electrostatic corrections

In cases where the system has no net charge and no net dipole moment, no specific tags need to be set and this section can be skipped.

Bulk

If the system has a net dipole or net charge, please follow the recommendations of this wiki page.

Surfaces

If the system has a net dipole moment, a combination of IDIPOL=1,2,3 and LDIPOL tags may be used. The former corrects the energies, while the latter corrects the potential and forces. Optionally, DIPOL may be set. The following options may be used to improve convergence for this case.

1. Use any of these tags only after pre-converging the orbitals without the LDIPOL tag

2. The center of charge should be set in the INCAR file (DIPOL= center of mass)

3. Ensure that the cell is sufficiently large to determine the dipole moment with sufficient accuracy (see DIPOL). If the cell is too small, the charge might slash through the vacuum, causing very slow convergence. Often convergence improves with the size of the supercell.

Warning: Surface calculations with a net charge result in total energies that do not converge. Relative energies may still be useful.

Wires

Not implemented.

Molecules

If the system has a net dipole moment, use the LDIPOL tag. The former corrects the energies, while the latter corrects the potential and forces. Optionally, DIPOL may be set.

Current limitations

For the current implementation, there are several restrictions; please read carefully:

  • Charged systems:
Quadrupole corrections are only correct for cubic supercells (this means that the calculated 1/L3 corrections are wrong for charged supercells if the supercell is non-cubic). In addition, we have found empirically that for charged systems with excess electrons (NELECT>NELECTneutral) more reliable results can be obtained if the energy after correction of the linear error (1/L) is plotted against 1/L3 to extrapolate results manually for L→∞. This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons.
  • Potential corrections are only possible for orthorhombic cells (at least the direction in which the potential is corrected must be orthogonal to the other two directions).

Step-by-step instructions

Using the dipole correction for slab calculations

In this section, we discuss step-by-step instructions to use the dipole corrections for slab calculations.

Step 1: Create a system which has enough vacuum on either side of the surface normal. An example for such a structure is shown below, for an fcc-Aluminium with a carbon adsorbed on one of its surface terminations. 

Al3C
1.0000000000000000
   2.8637824638055176    0.0000000000000000    0.0000000000000000
   1.4318912319027588    2.4801083645679673    0.0000000000000000
   0.0000000000000000    0.0000000000000000   20.0000000000000000
Al C
3 1
Direct
   0.8333333333333333    0.5000000000000000    0.3380865704891008
   0.1666666666666666    0.8333333333333334    0.4550000000000000
   0.4999999999999999    0.1666666666666667    0.5719134295108992
   0.4999999999999999    0.1666666666666667    0.6619134295108993

Note that the system has plenty of vacuum on either side. This empty space is important for the potential corrections needed for the LDIPOL tag.

Step 2: Switch on the dipole corrections to the energy, potential and forces. Optionally set the DIPOL 

LDIPOL    = T
IDIPOL    = 3
DIPOL     = 0.5 0.5 0.5

Step 3 (Optional): View the dipole moment for the system using the following bash command, 

grep dipolmoment OUTCAR | tail -1

In this example, we get the following output: 

 dipolmoment           0.000000      0.000000      0.128389 electrons x Angstroem

which refers to the dipole moment along the three axes. Consistent with the POSCAR used in this example, only the last axis has a non-zero dipole moment.

Related Tags and Sections

NELECT, EPSILON, DIPOL, IDIPOL, LDIPOL, LMONO, EFIELD

Examples that use this tag

References

  1. ↑ G. Makov and M. C. Payne, Phys. Rev. B 51, 4014 (1995).
  2. ↑ J. Neugebauer and M. Scheffler, Phys. Rev. B 46, 16067 (1992).
Retrieved from "https://www.vasp.at/wiki/index.php?title=Electrostatic_corrections&oldid=26051"