Diamond under pressure: Ab-initio calculations of the equation of state and optical phonon frequency revisited

The pressure–volume relationship and the zone-center optical phonon frequency of cubic diamond at pressures up to 600?GPa have been calculated based on density functional theory (DFT). Different methods (ranging from pseudopotential to all-electron approaches) and different approximations [local density approximation (LDA), generalized gradient approximation (GGA)] for the exchange-correlation (XC) energy have been applied. The results are compared to recent precise measurements up to 140?GPa. Possible implications for the experimental pressure determination based on the 1986 calibration of the ruby luminescence method are discussed.