poissonEquationParameters

	Core NanoLanguage elements
PeriodicAtomConfiguration	pseudoPotentialParameters

Name

poissonEquationParameters — Parameters controlling the evaluation of the Poisson equation.

Synopsis

Namespace: ATK.KohnSham

dictionary poissonEquationParameters(padding_factor)

Description

This function returns a dictionary with parameters associated with the electrostatic Poisson equation for molecules.

List of arguments

padding_factor

Electrostatic padding factor. Must be a non-negative number, which in general should not be decreased below the default value.

Default: 0.1

Usage examples

from ATK.KohnSham import *

molecule = MoleculeConfiguration(...)
padding = poissonEquationParameters(0.2)
method = KohnShamMethod(poisson_equation_parameters=padding)

scf = executeSelfConsistentCalculation(molecule,method)

Notes

Except for heterogeneous two-probe systems, ATK always uses a Fast Fourier Transform (FFT) method to solve the electrostatic Poisson equation. For molecules, however, there is no periodicity in the system; this is therefore introduced artificially by enclosing the molecule in a (large) unit cell enabling this method to be used.

The molecular unit cell is automatically chosen such that there are no matrix elements between atoms in adjacent cells. However, the electrostatic potential generated by the atoms often has a much longer reach than the pseudo-potentials and the basis sets themselves, and thus atoms in different cells can influence each other anyway. This leads to inaccurate results, and should be avoided as far as possible.

The size of the molecular unit cell can therefore be extended, or padded, by using the parameter described below.

Let $x_\mathrm{0}$ , $y_\mathrm{0}$ , and $z_\mathrm{0}$ denote the necessary lengths for avoiding interactions between the molecule and any of its repeated images. The molecular unit cell used for the calculation is then defined as having side lengths $x =(1+\mathrm{padding\_factor})\cdot x_0$ and correspondingly for $y$ and $z$ .

The default value 0.1 is known to work reasonably well even for molecules with rather large dipole moments. Still, for each case, it is recommended to test that a slightly higher value, say 0.15, does not lead to any significant changes in any of the results of the calculation. In general, the total energy changes only little with the padding factor, while the position of the HOMO level can be more sensitive. This is because the molecular energy levels are reported relative to the vacuum level, which is the value of the electrostatic potential at the edge of the molecular cell, and the vacuum level is more sensitive to the electrostatic interaction between the cells.


PeriodicAtomConfiguration		pseudoPotentialParameters