# External potential¶

## Examples¶

>>> # 2.5 eV/Ang along z:
>>> from gpaw.external import ConstantElectricField
>>> calc = GPAW(external=ConstantElectricField(2.5, [0, 0, 1]), ...)

class gpaw.external.ConstantElectricField(strength, direction=[0, 0, 1], tolerance=1e-07)[source]

External constant electric field.

strength: float

Field strength in V/Ang.

direction: vector

Polarisation direction.

>>> # Two point-charges:
>>> from gpaw.external import PointChargePotential
>>> pc = PointChargePotential([-1, 1], [[4.0, 4.0, 0.0], [4.0, 4.0, 10.0]])
>>> calc = GPAW(external=pc, ...)

class gpaw.external.PointChargePotential(charges, positions=None, rc=0.2, rc2=inf, width=1.0)[source]

Point-charge potential.

charges: list of float

Charges.

positions: (N, 3)-shaped array-like of float

Positions of charges in Angstrom. Can be set later.

rc: float

Inner cutoff for Coulomb potential in Angstrom.

rc2: float

Outer cutoff for Coulomb potential in Angstrom.

width: float

Width for cutoff function for Coulomb part.

For r < rc, 1 / r is replaced by a third order polynomial in r^2 that has matching value, first derivative, second derivative and integral.

For rc2 - width < r < rc2, 1 / r is multiplied by a smooth cutoff function (a third order polynomium in r).

You can also give rc a negative value. In that case, this formula is used:

(r^4 - rc^4) / (r^5 - |rc|^5)


for all values of r - no cutoff at rc2!