# 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.