DIPOL: Difference between revisions

Revision as of 12:07, 18 October 2023

DIPOL = [real array]

Description: specifies the center of the cell in direct lattice coordinates with respect to which the total dipole-moment in the cell is calculated.

The center of the cell w.r.t. which the total dipole-moment in the cell is calculated is specified as

DIPOL=R_x R_y R_z

where R_x, R_y and R_z are given in direct lattice coordinates.

Calculations using the dipole correction, i.e. using tags IDIPOL or LDIPOL require a definition of the center of the cell. Results of the computed dipole moment might differ for different positions. The reason for this difference is that the definition of the dipole 'destroys' the translational symmetry, i.e., the dipole is defined as

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

This measure will provide consistent values only if $\rho_{\rm ions+valence}$ drops to zero at some distance from $\mathbf R_{\rm center}$ . If this is not the case, the values are extremely sensitive with respect to changes in $\mathbf R_{\rm center}$ . In such cases, it might be beneficial to increase the size of the cell along the vacuum dimension (for surfaces) or for the entire cell (for isolated molecules).

Mind: 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)

Tip: For calculations of isolated molecules and surfaces with the dipole correction, use DIPOL as the center of mass of the atoms in your cell. Additionally, note that for surfaces, only the component normal to the surface is meaningful.

@@ Line 9: / Line 9: @@
 where '''R'''<sub>x</sub>, '''R'''<sub>y</sub> and '''R'''<sub>z</sub> are given in direct lattice coordinates.
-'''Mind''': the calculation of the dipole ({{TAG|IDIPOL}}=1-4) requires a definition of the center of the cell, and results might differ for different positions.
+Calculations using the dipole correction, i.e. using tags {{TAG|IDIPOL}} or {{TAG|LDIPOL}} require a definition of the center of the cell. Results of the computed dipole moment might differ for different positions. The reason for this difference is that the definition of the dipole 'destroys' the translational symmetry, i.e., the dipole is defined as
-You should use this option only for surfaces and isolated molecules.
-In this case, use the center of mass for the position. Additionally, mind that for surfaces, only the component normal to the surface is meaningful.
-The main problem is that the definition of the dipole 'destroys' the translational symmetry, i.e., the dipole is defined as
 :<math>
 \int ({\mathbf r}-{\mathbf R}_{\rm center}) \rho_{\rm ions+valence}({\mathbf r}) d^3 {\mathbf r}.
 </math>
+This measure will provide consistent values only if <math>\rho_{\rm ions+valence}</math> drops to zero at some distance from <math>\mathbf R_{\rm center}</math>. If this is
-Now, this makes only sense if <math>\rho_{\rm ions+valence}</math> drops to zero at some distance from <math>\mathbf R_{\rm center}</math>. If this is
+not the case, the values are extremely sensitive with respect to changes in <math>\mathbf R_{\rm center}</math>. In such cases, it might be beneficial to increase the size of the cell along the vacuum dimension (for surfaces) or for the entire cell (for isolated molecules).
-not the case, the values are extremely sensitive with respect to changes in <math>\mathbf R_{\rm center}</math>.
 {{NB|mind| 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)}}
+{{NB|tip| For calculations of isolated molecules and surfaces with the dipole correction, use {{TAG|DIPOL}} as the center of mass of the atoms in your cell. Additionally, note that for surfaces, only the component normal to the surface is meaningful.}}
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