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 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].

The following flags control the behaviour of VASP.

• NELECT, charged systems:
  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.

• DIPOL=R_x R_y R_z

where R_x, R_y and R_z 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

$\int ({\mathbf {r} }-{\mathbf {R} }_{\rm {center}})\rho _{\rm {ions+valence}}{\mathbf {r} }d^{3}{\mathbf {r} }.$

where ${\mathbf {R} }_{\rm {center}}$ is the position as defined by the 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).