A tip for calculating the energy curve for bulks

When the energy curve for bulk system is calculated as a function of the lattice parameter, a sudden change of the number of real space grids is a serious problem which produces an erratic discontinuity on the energy curve. In fact, we see the discontinuity in cases of 200 and 290 (Ryd) in Fig. 6 when the cutoff energy is fixed. The discontinuity occurs at the lattice parameter where the number of grids changes. To avoid the discontinuity on the energy curve, a keyword 'scf.Ngrid' is available.

    scf.Ngrid     32 32 32    # n1, n2, and n3 for a-, b-, and c-axes

When the number of grids is explicitly specified by the keyword, the axis is discretized by the number without depending on the keyword 'scf.energycutoff'. We see in Fig. 6 that the fixed grids with 32x32x32 gives a smooth curve, while the discontinuity is not so serious even in the cases of 'scf.energycutoff'.

Figure 6: The total energy of bcc iron as a function of the lattice parameter, where the experimental equilibrium lattice constant a$_0$ is 2.87 Å. A cubic unit cell including two atoms was considered. The input file is 'Febcc2.dat' in the directory 'work'.