We extend the powerful formalism of multichannel quantum defect theory combined with a frame transformation to ultracold atom-molecule collisions in magnetic fields. By solving the coupled-channel equations with hyperfine and Zeeman interactions omitted at short range, the extended theory enables a drastically simplified description of the intricate quantum dynamics of ultracold molecular collisions in terms of a small number of short-range parameters. We apply the formalism to ultracold $\mathrm{Mg}+\mathrm{NH}$ collisions in a magnetic field, achieving a ${10}^4$-fold reduction in computational effort.

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• Cold and ultracold molecules
• Scattering theory
• Ultracold collisions