  The lifetime of antiskyrmions at room temperature in a Mn-Pt-Sn tetragonal
Heusler material has been calculated using an atomic scale representation
including nearly a million spins. The evaluation of the pre-exponential factor
in the Arrhenius rate expression for this large system is made possible by an
implementation of harmonic transition state theory that avoids evaluation of
the eigenvalues of the Hessian matrix.The parameter values in the extended
Heisenberg Hamiltonian, including anisotropic Dzyaloshinskii-Moriya
interaction, are chosen to reproduce experimental observations [A. K. Nayak
$\it{et\,al.}$, Nature $\textbf{548}$, 561 (2017)], in particular the 150 nm
diameter. The calculated results are consistent with the long lifetime observed
in the laboratory and this exceptional stability of the antiskyrmions is found
to result from large activation energy for collapse due to strong exchange
coupling while the pre-exponential factor in the Arrhenius expression for the
lifetime is found to have a typical magnitude of 10$^{-12}$ s, despite the
large number of spins. The long lifetime is, therefore, found to result from
energetic effects rather than entropic effects in this system.
