Base compression parameters

At the heart of the FAST program is the suppression or reduction of linear dependence in the space of atomic orbital products used to describe the linear response (see papers 1 & 2 above). Dominant products are defined by diagonalisation of the Couloomb metric in the space of products of atomic orbitals and retention of a sub-set of eigenvectors with the largest eigenvalues. Several parameters control this process. Default values below have produced good results on test systems, but users are encouraged to experiment.

eigmin_local

(real): Defines the threshold for eigenvalues of the metric of orbital products on the same atom. Eigenvectors with smaller eigenvalues will be dropped in the computation of the sigma-matrices and spectrum.

Default: 1.0 E-3

eigmin_bilocal

(real): Defines the threshold for eigenvalues of the metric of orbital products on different atoms. Eigenvectors with smaller eigenvalues will be dropped in the computation of the sigma-matrix and spectrum.

Default: 1.0 E-4

use_eigmin_local_H

(boolean): If .true. use a special threshold for hydrogen local products.

Default: .false.

eigmin_local_H

(real): Threshold for hydrogen local products.

Default: eigmin_local/10

use_rcut

(boolean): If .true., drop orbital products for pairs of atoms with small geometrical orbital overlap, using criterion $r_{cut}$ set with the rcut keyword.

Default: .false.

rcut

(real): Allows suppression of further products, based on the thickness of the lens shaped region defined by the intersection of the largest orbitals on a pair atoms $A$ and $B$. All orbital products of a pair of atoms are dropped from the metric when

$$R_{max}(a) +R_{max}(b) - distance(a,b) < r_{cut},$$

where $R_{max}(a)$ (resp. $b$)is the radius of the support of all atomic orbitals on atom $A$ (resp. $B$).

Default: 3.0U where U is the unit of length for atomic positions the atoms stored in the siesta .DIM output file read by FAST.

write_all_eigens

(boolean): If .true. write all eigenvalues for all pairs of atoms in separate files. File names are constructed according to the rules :

For species the file name is construct

input_outputFileName //'_eigen_' // species //".txt")

For atoms pairs (e.g. atoms n1 and n2) the eigenvalues are stored in file

input_outputFileName//'_eigen_' // n1n2,//".txt"

Default: .false.