  Atomic forces formulation based on the Dirac-Kohn-Sham equation and flexible
(APW+lo/LAPW)+LO basis set is presented. The formulation was implemented in the
code FlapwMBPT and allows a user to easily switch between different basis
functions of the augmentation type (APW or LAPW) and between different kind of
local orbitals. Similar to the work (Phys.Rev.B 91 (2015) 035105), the
implementation takes into account small discontinuities of the wave functions,
density, and potential at the muffin-tin sphere boundaries. Applications to the
materials with strong relativistic effects, such as $\alpha$-Uranium,
PuCoGa$_{5}$, and FePt, demonstrate robustness of the method. Comparison of the
calculated forces with the ones obtained by numerical differentiation of the
free energy shows close agreement with deviations about 0.1\% or less.
