  A program has been designed for accurately generating a potential magnetic
field on a staggered grid by extrapolating the magnetic field normal to the
photospheric surface. The code first calculates a magnetic potential using the
Green's function method and then uses a finite differencing scheme to calculate
the magnetic field from the potential. A new finite differencing formula was
derived which accounts for grid staggering, it is shown that this formula gives
a numerical approximation that is closest to the real potential field. It is
also shown that extending the region over which normal photospheric field is
specified can improve the accuracy of the potential field produced. The program
is a FORTRAN 90 code that can be used to generate potential magnetic field
inputs for Lare3d and other MHD solvers that use a staggered grid for magnetic
field components. The program can be parallelised to run quickly over multiple
computing cores. The code and supporting description are provided in the
appendices and can also be found at
https://github.com/calboo/Potential_Field_Calculator.
