  We present quantum-theoretical studies of collisions between an open-shell
S-state atom and a ^2Pi-state molecule in the presence of a magnetic field. We
analyze the collisional Hamiltonian and discuss possible mechanisms for
inelastic collisions in such systems. The theory is applied to the collisions
of the nitrogen atom (^4S) with the OH molecule, with both collision partners
initially in fully spin-stretched (magnetically trappable) states, assuming
that the interaction takes place exclusively on the two high-spin (quintet)
potential energy surfaces. The surfaces for the quintet states are obtained
from spin-unrestricted coupled-cluster calculations with single, double, and
noniterative triple excitations. We find substantial inelasticity, arising from
strong couplings due to the anisotropy of the interaction potential and the
anisotropic spin-spin dipolar interaction. The mechanism involving the dipolar
interaction dominates for small magnetic field strengths and ultralow collision
energies, while the mechanism involving the potential anisotropy prevails when
the field strength is larger (above 100 G) or the collision energy is higher
(above 1 mK). The numerical results suggest that sympathetic cooling of
magnetically trapped OH by collisions with ultracold N atoms will not be
successful at higher temperatures.
