  In this work we provide an updated description of the Vertical Current
Approximation Nonlinear Force-Free Field (VCA-NLFFF) code, which is designed to
measure the evolution of the potential, nonpotential, free energies, and the
dissipated magnetic energies during solar flares. This code provides a
complementary and alternative method to existing traditional NLFFF codes. The
chief advantages of the VCA-NLFFF code over traditional NLFFF codes are the
circumvention of the unrealistic assumption of a force-free photosphere in the
magnetic field extrapolation method, the capability to minimize the
misalignment angles between observed coronal loops (or chromospheric fibril
structures) and theoretical model field lines, as well as computational speed.
In performance tests of the VCA-NLFFF code, by comparing with the NLFFF code of
Wiegelmann (2004), we find agreement in the potential, nonpotential, and free
energy within a factor of about 1.3, but the Wiegelmann code yields in the
average a factor of 2 lower flare energies. The VCA-NLFFF code is found to
detect decreases in flare energies in most X, M, and C-class flares. The
successful detection of energy decreases during a variety of flares with the
VCA-NLFFF code indicates that current-driven twisting and untwisting of the
magnetic field is an adequate model to quantify the storage of magnetic
energies in active regions and their dissipation during flares. - The VCA-NLFFF
code is also publicly available in the Solar SoftWare (SSW).
