Abstract
The equation of state used by the OPAL opacity code is based on a many-body activity expansion of the grand canonical partition function. Herein we give an overview of the method. This approach considers, from the outset, the basic Coulomb interaction between the electrons and nuclei in the system. Consequently, there is no need to factorize the free energy, make assertions about the effect of the plasma environment on bound states, or invoke a mechanism for truncating the internal partition function; as in free energy minimization methods. Key words: EQUATION OF STATE - PLASMAS
