where the first sum is over pairs of adjacent spins (every pair is counted once). The notation ⟨"ij"⟩ indicates that sites "i" and "j" are nearest neighbors. The magnetic moment is given by µ. Note that the sign in the second term of the Hamiltonian above should actually be positive because the electron's magnetic moment is antiparallel to its spin, but the negative term is used conventionally. The "configuration probability" is given by the Boltzmann distribution with inverse temperature β ≥ 0: