Monopole Dipole and Quadrupole Corrections

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 $L$ of the supercell. Using methods discussed by Makov et al. and Neugebauer et al., VASP can correct for the leading errors (in many details, we have taken a more general approach, though).

Summary of relevant INCAR tags

This section contains a summary of all the 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.

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
Any	No	No	None
3D	Yes	No	NELECT, LMONO, EPSILON
3D	No	Yes	DIPOL, IDIPOL, EPSILON
3D	Yes	Yes	NELECT, DIPOL, IDIPOL, EPSILON
2D	Yes	No	NELECT
2D	No	Yes	IDIPOL, LDIPOL, DIPOL
2D	Yes	Yes	NELECT
1D	Yes	No	NELECT
1D	No	Yes	Not implemented
1D	Yes	Yes	Not implemented
0D	Yes	No	NELECT, LMONO, LDIPOL
0D	No	Yes	LDIPOL
0D	Yes	Yes	NELECT, LMONO, LDIPOL

Tip: If an external electrostatic field is desired for slab, or molecular calculations, see EFIELD

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>NELECT_neutral) 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).

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.

Related Tags and Sections

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

Examples that use this tag

References

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