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Larmor precession is the precession of a magnetic moment about an applied magnetic field. For a spin with gyromagnetic ratio $\gamma$ and Hamiltonian $H=-\gamma \mathbf{B}\cdot \mathbf{S}$, its expectation obeys $d⟨\mathbf{S}angle/dt=\gamma ⟨\mathbf{S}angleimes\mathbf{B}$. A spin coherent state retains its coherent form during this rotation.

For particles with the same charge-to-mass ratio, rotationally invariant mutual central forces, and a weak uniform magnetic field, a frame with the displayed angular velocity cancels the velocity-dependent magnetic Lorentz force against the Coriolis force. The remaining rotational and magnetic terms are quadratic in $\mathbf{B}$. Thus, to first order in the field, the rotating-frame equations coincide with the zero-field equations. This is the classical dynamical basis of Larmor precession; equality is an approximation rather than an exact removal of the magnetic field.