Warm dense matter is a peculiar state with solid densities and temperatures 1 - 100 eV. Its ab initio description waits united efforts of quantum chemistry, condensed matter and plasma physics. We use the finite temperature Kohn-Sham density functional theory (a workhorse in this field) to study the pressure build-up with increase of electronic temperature in crystal and amorphous warm dense matter (WDM) gold. We compare the ab initio results with the effective ion-ion interaction model and reveal the possibility to separate the free electron contribution to the total pressure in WDM and to determine the corresponding degree of ionisation. For the sake of clarity, we try to describe our findings using the proper framework of statistical physics and briefly review the free energy models for WDM.