  We present first principles calculations of the dynamic susceptibility in
strained and doped ferromagnetic MnBi using time-dependent density functional
theory. In spite of being a metal, MnBi exhibits signatures of strong
correlation and a proper description in the framework of density functional
theory requires Hubbard corrections to the Mn $d$-orbitals. To permit
calculations of the dynamic susceptibility with Hubbard corrections applied to
the ground state electronic structure, we use a consistent rescaling of the
exchange-correlation kernel maintaining the delicate balance between the magnon
dispersion and the Stoner continuum. We find excellent agreement with the
experimentally observed magnon dispersion for pristine MnBi and show that the
material undergoes a phase transition to helical order under application of
either doping or strain. The presented methodology paves the way for future
LR-TDDFT studies of magnetic phase transitions, also for the wide range of
materials with pronounced static correlation effects that are not accounted for
at the LDA level.
