  In this work, we extend the source-free (SF) exchange correlation (XC)
functional developed by Sangeeta Sharma and co-workers to plane-wave density
functional theory (DFT) based on the projector augmented wave (PAW) method.
This constraint is implemented by the current authors within the VASP source
code, using a fast Poisson solver that capitalizes on the parallel
three-dimensional fast Fourier transforms (FFTs) implemented in VASP. Using
this modified XC functional, we explore the improved convergence behavior that
results from applying this constraint to the GGA-PBE+$U$+$J$ functional. In the
process, we compare the non-collinear magnetic ground state computed by each
functional and their SF counterpart for a select number of magnetic materials
in order to provide a metric for comparing with experimentally determined
magnetic orderings. We observe significantly improved agreement with
experimentally measured magnetic ground state structures after applying the
source-free constraint. Furthermore, we explore the importance of considering
probability current densities in spin polarized systems, even under no applied
field. We analyze the XC torque as well, in order to provide theoretical and
computational analyses of the net XC magnetic torque induced by the source-free
constraint. Along these lines, we highlight the importance of properly
considering the real-space integral of the source-free local magnetic XC field.
Our analysis on probability currents, net torque, and constant terms draws
additional links to the rich body of previous research on spin-current density
functional theory (SCDFT), and paves the way for future extensions and
corrections to the SF corrected XC functional.
