The red line represents the convergence criteria. The Y-axis is the energy difference between successive calculations. 2 K-point convergence for PBE functional. This process is performed with both the functionals. The essential step before performing a plane-wave basis set is to optimize the k-points and cut-off energy. All the calculations are performed with the spin-polarized option ON as Fe is a magnetic element. The core electrons were treated using “on the fly” generated (OTFG) ultra-soft pseudo-potential with a core radius of 2.4 Bohrs (1.27 Angstrom) generated with a panel of 8 valence electrons ( 3d6 4s2). Electronic convergence tolerance of 2E-06 was used for all the calculations. The exchange and correlation energies were calculated using the PBE and the PW 91 functional described within the generalized gradient approximation (GGA). All the calculations were performed using the CASTEP Simulation Package in Material Studio. We use the plane wave DFT calculations to evaluate the optimal lattice parameter for Fe bcc crystal structure.
A comparison between the results obtained from these functionals is made along with the computational costs associated with both of them. The PBE and PW91 functionals are expected to produce virtually identical results. Two functionals in the generalized gradient approximations, PBE and PW91, are used to derive these parameters. The Cambridge Serial Total Energy Package (CASTEP) software package, which uses plane-wave basis sets in order to analyze crystal structures, i s implemented to analyze the bcc lattices of Fe to determine the optimal parameters. Its optimal lattice parameters for this structure a re derived using Density-Functional Theory (DFT) methods. Iron (Fe) is known to have a bcc crystal structure at temperatures lower than 1100 K.
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