In a book about stat. mech., I read the following relaiton for magnetic system [tex]A = G + hM[/tex] where A is the Helmholtz energy, G is the Gibbs energy and h is the external magnetic field, M is the magetization. I know from thermodynamic, we have [tex]A = U - TS[/tex] or [tex]A = G - PV[/tex] so, [tex]hM = -PV[/tex] ? I don't understand what is [tex]hM[/tex]. If I know magnetization, external field and Gibbs energy, how to get Helmholtz energy?