Electrostatic corrections: Difference between revisions

From VASP Wiki
VisualWikitext
(Created page with '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 …')
 
 
(34 intermediate revisions by 7 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 $L$ of the supercell. Using methods discussed in Ref. [55,56] VASP can correct for the leading errors, but one should stress, that in many details, we have taken a more general approach than the one outlined in Ref. [55].  
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).  


The following flags control the behaviour of VASP.  
== 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 [[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.


* {{TAG|NELECT}}, charged systems:<br/> {{TAG|NELECT}} determines the total number of electrons in the system. The value may deviate from the default value, which is calculated assuming charge neutrality in the system. If NELECT differs from the default, an additional neutralizing background charge is applied by VASP.
1. Use any of these tags only after pre-converging the orbitals without the {{TAG|LDIPOL}} tag


* {{TAG|DIPOL}}='''R'''<sub>x</sub> '''R'''<sub>y</sub> '''R'''<sub>z</sub>
2. The center of charge should be set in the {{FILE|INCAR}} file ({{TAG|DIPOL}}= center of mass)
:where '''R'''<sub>x</sub>, '''R'''<sub>y</sub> and '''R'''<sub>z</sub> specify the center of the cell in direct, fractional coordinates.


This tag determines the center of the net charge distribution. The dipole is defined as
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.
:<math>
{{NB|warning|Surface calculations with a net charge result in total energies that do not converge. Relative energies may still be useful.}}
\int ({\mathbf r}-{\mathbf R}_{\rm center}) \rho_{\rm ions+valence}{\mathbf r} d^3 {\mathbf r}.
 
</math>
=== Wires ===
where <math>{\mathbf R}_{\rm center}</math> is the position as defined by the {{TAG|DIPOL}} tag. If the flag is not set, VASP determines, where the charge density averaged over one plane drops to a minimum and calculates the center of the charge distribution by adding half of the lattice vector perpendicular to the plane where the charge density has a minimum (this is a rather reliable approach for orthorhombic cells).
Not implemented.
 
=== Molecules ===
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.
 
== 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/''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).
 
== 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 ==
{{TAG|NELECT}},
{{TAG|EPSILON}},
{{TAG|DIPOL}},
{{TAG|IDIPOL}},
{{TAG|LDIPOL}},
{{TAG|LMONO}},
{{TAG|EFIELD}}
 
{{sc|Monopole Dipole and Quadrupole corrections|Examples|Examples that use this tag}}
 
== References ==
<references>
<ref name="Makov95">[http://dx.doi.org/10.1103/PhysRevB.51.4014 G. Makov and M. C. Payne, Phys. Rev. B 51, 4014 (1995).]</ref>
<ref name="Neugebauer92">[http://dx.doi.org/10.1103/PhysRevB.46.16067 J. Neugebauer and M. Scheffler, Phys. Rev. B 46, 16067 (1992).]</ref>
</references>
 
----
 
[[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"