A derivation of the isothermal quantum hydrodynamic equations using entropy minimization

Dedicated to Prof. Herbert Gajewski in occasion of his 65th birthday Isothermal quantum hydrodynamic equations of order O($\hbar$2) using the quantum entropy minimization method recently developed by Degond and Ringhofer are derived. The equations have the form of the usual quantum hydrodynamic model including a correction term of order O($\hbar$2 which involves the vorticity. If the initial vorticity is of order O($\hbar$4, the standard model is obtained up to order . The derivation is based on a careful expansion of the quantum equilibrium obtained from the entropy minimization in powers of $\hbar$2.