ON THE STRUCTURAL-PROPERTIES OF NACL - AN ABINITIO STUDY OF THE B1-B2PHASE-TRANSITION

The structural properties and the phase transition between the B1 (roc k salt) and B2 (CsCl) phases of NaCl have been investigated at an ab i nitio level. Total energy curves, obtained with periodic Hartree-Fock (HF) calculations, have been corrected a posteriori by integrating the HF charge density according to the correlation-only density-functiona l-like formulae proposed by Colle and Salvetti (CS75) in 1975 and by P erdew in 1986 (P86) and 1991 (P91). The three functionals are 'non-loc al', containing terms that depend on the gradient of the electronic de nsity. The correlation correction brings the HF data for the lattice e nergy (LE), lattice parameter (a(o)) and bulk modulus (B) into good ag reement with experiment, the best performance being obtained with the most recent functional, P91 (the percentage error is +0.3%, -0.5% and +5.6% respectively for LE, a(o) and B). The HF transition pressure (39 .2GPa) is slightly modified by the CS75 correction (37.2); P86 and P91 functionals reduce the computed value to 30.1 GPa and 28.9 GPa respec tively, in better agreement with experiment (30 GPa).