KERNEL TRUNCATION/LTRUNCATE: Difference between revisions

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Setting {{TAG|KERNEL_TRUNCATION/LTRUNCATE}} = T  switches on the Coulomb-kernel-truncation method{{cite|vijay:prb:2025}}{{cite|rozzi:prb:2006}}{{cite|sohier:prb:2017}}. It effectively removes interactions with periodic replicas in non-periodic directions. In other words, the interactions are removed along the surface normal for [[2D materials]], and along all directions for 0D systems, i.e. for isolated atoms and molecules.
Setting {{TAG|KERNEL_TRUNCATION/LTRUNCATE}} = T  switches on the Coulomb-kernel-truncation method{{cite|vijay:prb:2025}}{{cite|rozzi:prb:2006}}{{cite|sohier:prb:2017}}. It effectively removes interactions with periodic replicas in non-periodic directions. In other words, the interactions are removed along the surface normal for [[2D materials]], and along all directions for 0D systems, i.e. for isolated atoms and molecules.
{{NB|important| Slabs must be centered in the unit cell.}}
 
In the simplest implementation of the Coulomb-kernel-truncation method ({{TAG|KERNEL_TRUNCATION/LCOARSEN|F}}), the computational cell provided in the {{FILE|POSCAR}} file is internally padded by an additional vacuum (see {{TAG|KERNEL_TRUNCATION/IPAD}}). This implies increasing the [[Energy cutoff and FFT meshes|FFT-grid sizes]] by a certain factor and thus leads to a significant increase in computational cost.
In the simplest implementation of the Coulomb-kernel-truncation method ({{TAG|KERNEL_TRUNCATION/LCOARSEN|F}}), the computational cell provided in the {{FILE|POSCAR}} file is internally padded by an additional vacuum (see {{TAG|KERNEL_TRUNCATION/IPAD}}). This implies increasing the [[Energy cutoff and FFT meshes|FFT-grid sizes]] by a certain factor and thus leads to a significant increase in computational cost.
{{NB|tip|Use the {{TAG|KERNEL_TRUNCATION/LCOARSEN|T}} to avoid the increased [[Energy cutoff and FFT meshes|FFT-grid sizes]].}}
{{NB|tip|Use the {{TAG|KERNEL_TRUNCATION/LCOARSEN|T}} to avoid the increased [[Energy cutoff and FFT meshes|FFT-grid sizes]].}}
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*This tag is only available as of VASP.6.5.0.}}
*This tag is only available as of VASP.6.5.0.}}
Detailed information about the setting are documented on respective related tags.
Detailed information about the setting are documented on respective related tags.
{{NB|warning|When padding is used, the vaccum is added on the edges of the cell, as such it is very important that the motif is centered in the simulation box. If you encounter problems using Coulomb truncation with padding, try the same calculations without padding (see examples bellow).}}
== Example ==
<pre>
KERNEL_TRUNCATION {
    LTRUNCATE      = T
    IDIMENSIONALITY = 2
    ISURFACE        = 3
    IPAD            = 2
    FACTOR          = 1
}
</pre>
In this case an additional empty cell is added along the z direciton as padding. The Coulomb interaction is truncated beyond a z length. This ensures maximum usage of the simulation box.
<pre>
KERNEL_TRUNCATION {
    LTRUNCATE      = T
    IDIMENSIONALITY = 2
    ISURFACE        = 3
    IPAD            = 1
    FACTOR          = 0.5
}
</pre>
This setup corresponds to truncating the Coulomb interaction along the surface normal (z-direction) for a [[2D material]], using no vacuum padding and a truncation length of z/2. In this case, half of the simulation box is effectively unused, but the algorithm remains simpler. This configuration can be useful for debugging purposes.


== Related tags and articles ==
== Related tags and articles ==

Revision as of 11:21, 17 March 2026

KERNEL_TRUNCATION/LTRUNCATE = .True. | .False.
Default: KERNEL_TRUNCATION/LTRUNCATE = .False. 

Description: Truncates the Coulomb kernel to remove electrostatic interactions along non-periodic dimensions.

Setting KERNEL_TRUNCATION/LTRUNCATE = T switches on the Coulomb-kernel-truncation method[1][2][3]. It effectively removes interactions with periodic replicas in non-periodic directions. In other words, the interactions are removed along the surface normal for 2D materials, and along all directions for 0D systems, i.e. for isolated atoms and molecules.

In the simplest implementation of the Coulomb-kernel-truncation method (KERNEL_TRUNCATION/LCOARSEN = F), the computational cell provided in the POSCAR file is internally padded by an additional vacuum (see KERNEL_TRUNCATION/IPAD). This implies increasing the FFT-grid sizes by a certain factor and thus leads to a significant increase in computational cost.

Tip: Use the KERNEL_TRUNCATION/LCOARSEN = T to avoid the increased FFT-grid sizes.
Mind:
  • KERNEL_TRUNCATION/LTRUNCATE acts as a "super-tag", i.e. unless this tag is switched on further options in KERNEL_TRUNCATION will be ignored.
  • This tag is only available as of VASP.6.5.0.

Detailed information about the setting are documented on respective related tags.

Warning: When padding is used, the vaccum is added on the edges of the cell, as such it is very important that the motif is centered in the simulation box. If you encounter problems using Coulomb truncation with padding, try the same calculations without padding (see examples bellow).

Example

KERNEL_TRUNCATION {
     LTRUNCATE       = T
     IDIMENSIONALITY = 2
     ISURFACE        = 3
     IPAD            = 2
     FACTOR          = 1
}

In this case an additional empty cell is added along the z direciton as padding. The Coulomb interaction is truncated beyond a z length. This ensures maximum usage of the simulation box. 

KERNEL_TRUNCATION {
     LTRUNCATE       = T
     IDIMENSIONALITY = 2
     ISURFACE        = 3
     IPAD            = 1
     FACTOR          = 0.5
}

This setup corresponds to truncating the Coulomb interaction along the surface normal (z-direction) for a 2D material, using no vacuum padding and a truncation length of z/2. In this case, half of the simulation box is effectively unused, but the algorithm remains simpler. This configuration can be useful for debugging purposes.

Related tags and articles

KERNEL_TRUNCATION/LCOARSEN, KERNEL_TRUNCATION/IDIMENSIONALITY, KERNEL_TRUNCATION/ISURFACE, KERNEL_TRUNCATION/FACTOR, KERNEL_TRUNCATION/IPAD

References

  1. ↑ S. Vijay, M. Schlipf, H. Miranda, F. Karsai, M. Kaltak, M. Marsman, and G. Kresse, Efficient periodic density functional theory calculations of charged molecules and surfaces using Coulomb kernel truncation, Phys. Rev. B 112, 045409 (2025).
  2. ↑ C. A. Rozzi, D. Varsano, A. Marini, E. K. Gross, A. J. Rubio, Phys. Rev. B 73, 20511 (2006).
  3. ↑ T. Sohier, M. Calandra, and F. Mauri, Phys. Rev. B 96, 75448 (2017).
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