KERNEL TRUNCATION/LCOARSEN

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

Description: Coarsen the charge density before padding to speed up the computation of the local potential in the Coulomb-kernel-truncation method.

The Coulomb-kernel-truncation method (KERNEL_TRUNCATION/LTRUNCATE) modifies the electrostatic potential to remove spurious interactions between periodic images in systems with reduced dimensionality (e.g. molecules or surfaces). KERNEL_TRUNCATION/LCOARSEN = T avoids significantly increasing FFT-grid sizes by using a coarsen-before-padding approach:

The full electrostatic potential is first computed under standard 3D periodic boundary conditions.
A coarse representation of the charge density on a reduced FFT-grid which is constructed by retaining only the long-wavelength (low-|G|) components responsible for long-range interactions.
The truncated Coulomb kernel is applied only to this coarse density using a padded grid (see KERNEL_TRUNCATION/IPAD).
The final potential is obtained by subtracting the long-range periodic contribution and replacing it with the correctly truncated one.

This procedure exploits the fact that long-range electrostatics depends only on low-frequency components of the charge density, allowing the expensive padded FFTs to be performed on a much smaller grid with minimal loss of accuracy.

Mind: If KERNEL_TRUNCATION/LTRUNCATE = F, all other KERNEL_TRUNCATION tags including KERNEL_TRUNCATION/LCOARSEN are ignored.

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