  In a recent Letter [1], Wernsdorfer et al. report an experimental study of a
Mn12 molecular wheel which shows essentially identical behavior to the Mn12
wheel studied by Ramsey et al. [2]. In their Letter, Wernsdorfer et al. use the
same model of a dimer of two exchange-coupled spins used in [2] as a basis to
extend the study of the influence of the Dzyaloshinskii-Moriya (DM) interaction
on the quantum tunneling of the magnetization of this system; in particular,
they show that a tilt of the DM vector away from the uniaxial anisotropy axis
can account for the asymmetric nature of the quantum interference minima
associated with resonances between states of opposite parity, e.g., k = 1(A).
We want to stress that the inclusion of DM interactions in a system with
inversion symmetry cannot mix states of opposite parity; i.e., the parity
operator commutes with the Hamiltonian. Consequently, the use by Wernsdorfer et
al. of a single DM vector in a centrosymmetric dimer is strictly forbidden
since it implicitly violates parity conservation. The authors correctly point
out that the lack of an inversion center between each pair of manganese ions on
the wheel justifies the possibility of local DM interactions, even though the
complete molecule has an inversion center. However, these local DM interactions
must also satisfy the molecular inversion symmetry; i.e., they cannot mix
states of opposite parity.We agree that such DM interactions are not always
completely innocuous; e.g., they can mix spin states having the same parity.
Indeed, in kagome systems [3] (cited in [1]), this can lead to weak
ferromagnetism. Nevertheless, the inversion symmetry of the lattice is
preserved and parity is still conserved.
