Consequently, LDA is generally related to functionals, which are based on HEG approximation. LDA are implemented in realistic systems measurements and calculations (for example, molecules and solids). In a general view, for a spin-unpolarized system, “LDA for the exchange-correlation energy is the following where ? refers to the electronic density and ?xc, the exchange-correlation energy concentration, is a function of the density. The exchange-correlation energy is split into exchange and correlation terms in a linear view: thus separate expressions for Ex and Ec are defined. Separate expressions known for the correlation density lead to numerous different approximations for ?c”. (Parr and Weitao, 1994) LDA is a part of any approximate exchange-correlation functional. Its main function is to “replicate direct results of the HEG for non-varying densities” (Parr and Weitao 1994, p. 35). As a result, LDA is often mentioned as an explicit component of such kind of functional. The Perdew-Burke-Ernzerhof (PBE) Modern scientists and researchers are greatly concerned about updating GGA functionals in their implementation in molecules and solids. ...

Currently, accents in “generalized gradient approximations are shifted toward the description of free-atom energies” (Perdew et al, 2008). A group of scientists, Perdew et al (2008) have transformed and “adapted the Perdew-Burke-Ernzerhof generalized gradient approximation that updates equilibrium characteristics of dense solids and their surfaces” (Perdew et al, 2008). It is relevant to underline that the Perdew-Burke-Ernzerhof is a widely-used tool in solid state calculations. Consequently, a relevant note is that PBE is currently transformed with regard to GGA implementing both the density and its gradient at every space point (Perdew et al, 2008). If to correlate both concepts GGA and PBE, it will be clearly seen that GCA counterbalances “computational efficiency, numerical accuracy, and reliability” (Perdew et al 2008, p. 4068). Moreover, “PBE refers to the demands of quantum chemistry and solid-state physics” (Perdew et al 2008, p. 4068). Modern scientists think that it is necessary to ‘widen horizons’ of PBE implementation. Thus during the last decade it has been found out that “PBE reduces the chronic overbinding of the local spin density approximation (LSDA) [1], but, while LSDA often slightly underestimates equilibrium lattice constants by about 1%, PBE usually overestimates them by about the same amount” (Perdew et al 2008, p. 4070). Moreover, another important point of PBE properties is its equilibrium properties (e.g. bulk moduli, phonon frequencies, magnetism, and ferroelectricity). The abovementioned properties are susceptible to the lattice constant; thus they are ‘overcorrected’ by PBE (Perdew et al, 2008). Moreover, in spite of the fact that LSDA defines low
...Show more