NUMBER OF BANDS

A.K.A. – The importance of calculating the best occupied subspace

For systems with a gap, always use the minimum number of bands

For Mauri and Grassmann we see poorer results for more bands thyan the minimum. With Grassmann_metal we see no degradation. The increased time in the bands is made up for by the increased convergence.

Examples

Systems with a gap

Ex.1 2 atom cell of GaAs 10 k-points

For this system with 4 occupied bands, there is no difference between 4 and 8 bands.

R bands is more efficient though.

Ex.2 12 atoms. 3 layers GaAs – 3 layers InAs

One can see the worsened convergence by not using the minimum number of bands.

Ex.3 20 C and 2 H

In this example, a small value of 3 for the submatrix cutoff and too many bands significantly

degrades the convergence. With the number of minimum bands, the convergence has some problems but there is significant improvement.

Metals – fractional occupancies

Ex.4 5 atoms 50 electrons: 5 layers of Ni 5 vacuum in (100)

Ex.5 6 atoms 60 electrons: 6 layers of Ni and 6 vacuum in (100)

Using the Grassmann_metal option for these metallic systems, the number of bands is not as crucial.