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 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. .
The following flags control the behaviour of VASP.
NELECT determines the total number of electrons in the system (see Sec. 6.35). 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.
In this case, however, the energy converges very slowly with respect to the size of the super cell. The required first order correction to the energy is given by
It is important to emphasize that the total energy can not be corrected for charged slabs, since a charged slab results in an electrostatic potential that grows linearly with the distance from the slab (corresponding to a fixed electrostatic field). It is fairly simple to show that as a result of the interaction between the charged slab and the compensating background, the total energy depends linearly on the width of the vacuum. Presently, no simple a posteriori correction scheme is known or implemented in VASP. Total energies from charged slab calculations are hence useless, and can not be used to determine relative energies.
Note: If you are not convinced, simply vary the vacuum width and draw the energy versus the vacuum width.
For systems with a net dipole moment, the energy also converges slowly with respect to the size of the super cell. The dipole corrections (and quadrupole corrections for charged systems) fall off like . Both corrections, dipole and quadrupole for charged systems, will be calculated and added to the total energy if the IDIPOL flag is set.
Note: strictly speaking quadrupole corrections is not the proper wording. The relevant quantity is
The flag EPSILON can be used to supply the dielectric constant of the medium. VASP uses this flag only to scale the calculated monopole and dipole corrections. EPSILON defaults to 1, which is the proper value for isolated atoms and molcules. For solids, the screening properties can and should be determined using the linear response routines of VASP (Sec. 6.72.4). Ionic contributions to the dielectric tensor should be included, if the ions are allowed to relax. Ionic contributions to the dielectric tensor can be calculated using IBRION=8 (Sec. 6.72.6).
If set in the INCAR file monopole/dipole and quadrupole corrections will be calculated. There are four possible settings for IDIPOL
IDIPOL = 1-4For 1 to 3, the dipole moment will be calculated only parallel to the direction of the first, second or third lattice vector. The corrections for the total energy are calculated as the energy difference between a monopole/dipole and quadrupole in the current supercell and the same dipole placed in a super cell with the corresponding lattice vector approaching infinity. This flag should be used for slab calculations.
For IDIPOL=4 the full dipole moment in all directions will be calculated, and the corrections to the total energy are calculated as the energy difference between a monopole/dipole/quadrupole in the current supercell and the same monopole/dipole/quadrupole placed in a vacuum, use this flag for calculations for isolated molecules.
DIPOL = center of cell (in direct, fractional coordinates)This tag determines the center of the net charge distribution. The dipole is defined as
These tags switch on the potential correction mode. Due to the periodic boundary conditions, not only the total energy converges slowly with respect to the size of the supercell, but also the potential and the forces are affected by finite size errors. This effect can be counterbalanced by setting LDIPOL=.TRUE. (dipole corrections) and/or LMONO=.TRUE. (monopole corrections) in the INCAR file. For LDIPOL=.TRUE.,a linear and for LMONO=.TRUE., a quadratic electrostatic potential is added to the local potential, correcting the errors introduced by the periodic boundary conditions. This is in the spirit of Ref.  (but more general and the total energy has been correctly implemented right away). The biggest advantage of this mode is that leading errors in the forces are corrected, and that the work-function can be evaluated for asymmetric slabs. The disadvantage is that the convergence to the electronic groundstate might slow down considerably (i.e. more electronic iterations might be required to obtain the required precision). It is recommended to use this mode only after pre-converging the orbitals without the LDIPOL flag, and the center of charge should be set in the INCAR file (DIPOL= center of mass). The user must also ensure that the cell is sufficiently large to determine the dipole moment with sufficient accuracy. If the cell is too small, charge might swap through the vacuum, causing very slow convergence (often convergence improves with the size of the supercell).
It is possible to apply an external electrostatic field in slab, or molecular calculations. Presently only a single value can be supplied and the field is applied in the direction selected by IDIPOL (1-3). The field is supplied in units of eV/Å. Dipole corrections (LDIPOL=.TRUE.) can and should be turned on to avoid interactions between the periodically repeated images.
Quadrupole corrections are only correct for cubic supercells (this means that the calculated 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 (NELECTNELECT ) more reliable results can be obtained if the energy after correction of the linear error () is plotted against to extrapolate results manually for . This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons.