A central concept in NMR is the precession of the spin magnetization around the magnetic field at the nucleus, with the angular frequency  where  relates to the oscillation frequency  and B is the magnitude of the field. This means that the spin magnetization, which is proportional to the sum of the spin vectors of nuclei in magnetically equivalent sites (the expectation value of the spin vector in quantum mechanics), moves on a cone around the B field. This is analogous to the precessional motion of the axis of a tilted spinning top around the gravitational field. In quantum mechanics,  is the Bohr frequency  of the  and  expectation values. Precession of non-equilibrium magnetization in the applied magnetic field B occurs with the Larmor frequency  without change in the populations of the energy levels because energy is constant (time-independent Hamiltonian).