More Technical Settings¶

There are of course many other settings influencing the precision and performance. Usually the user does not need to care about them.

Linear Scaling¶

Tails
   Bas float
End

Tails

Type: Block
Description: Ignore function tails.

Bas

Type: Float
Default value: 1e-06
GUI name: Basis functions
Description: Cut off the basis functions when smaller than the specified threshold.

Dependency¶

Dependency
   AllowBasisDependency Yes/No
   Basis float
   Core float
   CoreValence float
   Fit float
End

Dependency

Type: Block
Description: Criteria for linear dependency of the basis and fit set

AllowBasisDependency

Type: Bool
Default value: Yes
Description: Project out the dependent part of the basis set (associated with small eigenvalues of the overlap matrix).

Basis

Type: Float
Default value: 1e-08
GUI name: Dependency criterion
Description: Criteria for linear dependency of the basis: smallest eigenvalue of the overlap matrix of normalized Bloch functions.

Core

Type: Float
Default value: 0.8
Description: The program verifies that the frozen core approximation is reasonable, by checking the smallest eigen value of the overlap matrix of the core (Bloch) orbitals (which should ideally be one) is bigger than this criterion.

CoreValence

Type: Float
Default value: 1e-05
Description: Criterion for dependency of the core functions on the valence basis. The maximum overlap between any two normalized functions in the two respective function spaces should not exceed 1.0-corevalence

Fit

Type: Float
Default value: 5e-06
Description: Criterion for dependency of the total set of fit functions. The value monitored is the smallest eigenvalue of the overlap matrix of normalized Bloch sums of symmetrized fit functions.

Screening¶

Band performs many lattice summations which are in practice truncated. The two prime examples are the construction of the Bloch basis and the calculation of the solvation potential. The precision of the lattice summations is controlled by the Screening key

Screening
   CutOff float
   DMadel float
   NoDirectionalScreening Yes/No
   RCelx float
   RMadel float
End

Screening

Type: Block
Description: For the periodic solvation potential and for the old (not default anymore) fitting method, BAND performs lattice summations which are in practice truncated. The precision of the lattice summations is controlled by the options in this block.

CutOff

Type: Float
Description: Criterion for negligibility of tails in the construction of Bloch sums. Default depends on Accuracy.

DMadel

Type: Float
Description: One of the parameters that define the screening of Coulomb-potentials in lattice sums. Depends by default on Accuracy, rmadel, and rcelx. One should consult the literature for more information

NoDirectionalScreening

Type: Bool
Description: Real space lattice sums of slowly (or non-) convergent terms, such as the Coulomb potential, are computed by a screening technique. In previous releases, the screening was applied to all (long-range) Coulomb expressions. Screening is only applied in the periodicity directions. This key restores the original situation: screening in all directions

RCelx

Type: Float
Description: Max. distance of lattice site from which tails of atomic functions will be taken into account for the Bloch sums. Default depends on Accuracy.

RMadel

Type: Float
Description: One of the parameters that define screening of the Coulomb potentials in lattice summations. Depends by default on Accuracy, dmadel, rcelx. One should consult the literature for more information.

Direct (on the fly) calculation of basis and fit¶

BAND usually calculates basis functions and theirs derivatives on the fly. However, for small bulk systems it can be faster to write the information to disk. Then one can set the DirectBas key to false. (Default = true)

Programmer
   DirectBas bool
End

Fermi energy search¶

Fermi
   Delta float
   Eps float
   MaxTry integer
End

Fermi

Type: Block
Description: Technical parameter used in determining the Fermi energy, which is carried out at each cycle of the SCF procedure.

Delta

Type: Float
Default value: 0.0001
Description: Convergence criterion: upper and lower bounds for the Fermi energy and the corresponding integrated charge volumes must be equal within delta.

Eps

Type: Float
Default value: 1e-10
Description: After convergence of the Fermi energy search procedure, a final estimate is defined by interpolation and the corresponding integrated charge volume is tested. It should be exact, to machine precision. Tested is that it deviates not more than eps.

MaxTry

Type: Integer
Default value: 15
Description: Maximum number of attempts to locate the Fermi energy. The procedure is iterative in nature, narrowing the energy band in which the Fermi energy must lie, between an upper and a lower bound. If the procedure has not converged sufficiently within MaxTry iterations, the program takes a reasonable value and constructs the charge density by interpolation between the functions corresponding to the last used upper and lower bounds for the Fermi energy.

Block size¶

CPVector integer

CPVector

Type: Integer
Default value: 128
GUI name: Vectorlength (blocksize)
Description: The code is vectorized and this key can be used to set the vector length

KGrpX integer

KGrpX

Type: Integer
Default value: 5
GUI name: Number of K-points at once
Description: Absolute upper bound on the number of k-points processed together. This only affects the computational performance.