POT4A: a program for the direct solution of Poisson’s equation in complex geometries. The program solves Poisson’s equation in a rectangular region. Electrodes and dielectric interfaces can be included anywhere in the region. An electrode is defined as a line segment along which the potential is held at a given value. A dielectric interface is defined as a line across which the normal component of the electric displacement is held constant. The boundary conditions on the edge of the rectangle can be given value, zero normal gradient or periodic in the x-direction. In the y-direction, in addition to these conditions, a mixed boundary condition of the form aφ+b∂φ ∂y=c(x) and an open boundary condition, where ∂φ ∂y→0 as |y|→∞ are included. The program was developed to solve problems in electrostatics but can also be applied in other fields (Source: http://cpc.cs.qub.ac.uk/summaries/)