Here, the factor 2 m could, in principle, be replaced by any positive number, which shows that the symmetrizer is not always unique. The choice here is such that the expression ∗ u Hu is proportional to the physical energy density of the system. Notice, however, that for the massless case m = 0 one must replace 2 m = 0 by a positive constant in order for the symmetrizer to be positive definite.