KERNEL TRUNCATION/FACTOR

KERNEL_TRUNCATION/FACTOR = real 

Default: KERNEL_TRUNCATION/FACTOR	= $\sqrt{3}$	if KERNEL_TRUNCATION/IDIMENSIONALITY = 0
	= 1	if KERNEL_TRUNCATION/IDIMENSIONALITY = 2

Description: KERNEL_TRUNCATION/FACTOR determines the spatial extent of the truncated Coulomb interaction relative to the full cell dimension along the truncation direction.

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This tag defines the cutoff distance of the Coulomb kernel truncation boundary. It is expressed as a fraction of the simulation cell length along the truncated axis (e.g., the surface normal for 2D systems).

Mind: KERNEL_TRUNCATION/LTRUNCATE must be set to .TRUE. for KERNEL_TRUNCATION/FACTOR to take effect.

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/LTRUNCATE, KERNEL_TRUNCATION/IDIMENSIONALITY, KERNEL_TRUNCATION/LCOARSEN, KERNEL_TRUNCATION/IPAD, KERNEL_TRUNCATION/ISURFACE