Ab initiocalculations of static lattice properties for NaCl and a test of the Decker equation of state

Calculations of the equilibrium volume and bulk modulus at the equilibrium volume are made for Gordon-Kim based theories of NaCl and used in the Decker approach for calculating the equation of state. Generally the resultant ``Decker equation of state'' is in excellent agreement with the exact calculations. However, for one theory a 9% discrepancy is obtained for the highest B1-phase pressures, i.e., p=30 GPa. Results of the application of this analysis to recent linear augmented-plane-wave calculations, which make no approximation beyond the local-density approximation, are also presented and give added support to the validity of the Decker procedure. It is also shown that a five-parameter Birch formula, where tested rigorously, is an excellent representation of crystal-energy versus volume calculations. Finally, results of the second pressure derivative of the bulk modulus, evaluated at zero pressure, are found to be in fair agreement with a recent experimental determination.