Generalized gradient approximation correlation wave functions

The wave functions are expended in a plane wave basis set, and the effective potential of ions is described by ultrasoft pseudo potential. The generalized gradient approximation (GGA)-PW91, and local gradient-corrected exchange-correlation functional (LDA)-CAPZ are used for the exchange-correlation functional. 

These back-of-the-envelope calculations may be compared with the results of full-potential electronic-structure calculations of the FLAPW type (WIEN2k) based on the generalized gradient approximation (GGA) for the exchange-correlation potential. On purpose, we use a very accurate DFT method. Figure 3.1 shows the results of these quantum-mechanical calculations as a plot of the total energy as a function of the volume per formula unit for all three structure types. The calculations are based on 482/266/289 plane waves for... 

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