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.
Tails Bas float End
Type: Block Description: Ignore function tails.
Type: Float Default value: 1e-06 GUI name: Basis functions Description: Cut off the basis functions when smaller than the specified threshold.
Dependency Basis float Core float CoreValence float Fit float End
Type: Block Description: Criteria for linear dependency of the basis and fit set
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.
Type: Float Default value: 0.98 Description: The program verifies that the frozen core approximation is reasonable, by checking the smallest value of the overlap matrix of the core (Bloch) orbitals against this criterion.
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
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.
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 CutOff float DMadel float NoDirectionalScreening [True | False] RCelx float RMadel float End
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.
Type: Float Description: Criterion for negligibility of tails in the construction of Bloch sums. Default depends on Accuracy.
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
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
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.
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
Type: Block Description: Technical parameter used in determining the Fermi energy, which is carried out at each cycle of the SCF procedure.
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.
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.
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.
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
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.