The Hamiltonian for particle in an EM field is H = 1/2m (p - qA)^2 + q phi If we take the cross-terms, which corresponds to the paramagnetic term, we have [tex] H para = -q/2m * (p.A + A.p ) = iqh/2m * (\nabla .A + A.\nabla) [/tex] What I do not understand is how this simplifies into [tex] iqh/m * A.\nabla [/tex]? assuming that [tex] \nabla .A = 0[/tex] (i.e. Coulomb gauge). Why does the factor of 1/2 disappears? I'm only a first year undergraduate and I'm learning this on my own. I will appreciate it if you give a fuller answer. Thank you.