# 1.6. Atomic orbital calculation

In this section, I teach how to perform atomic orbital (AO)
calculations. AO calculations are more efficient, especially for large
systems, but the accuracy of an AO basis depends on the chemical
environment and the atomic configuration. To perform self-consistent,
DOS or band structure calculations using the AO basis, define the
keyword `LCAO.status`

as true. For example, let’s recompute the ground
state density of silicon using AO. Copy the following input file and
save it to a text file named `si_lcao_scf.txt`

.

```
info.calculationType = 'self-consistent'
info.savepath = 'results/si_lcao_scf';
atom.element = [1 1]
atom.xyz = 10.25*[0 0 0;0.25 0.25 0.25]
domain.latvec = 10.25*[0.0 0.5 0.5;0.5 0.0 0.5;0.5 0.5 0.0]
domain.lowres = 0.5
element.species = 'si'
element.path = './Si_TM_LDA.mat'
LCAO.status = 1
kpoint.gridn = [4,4,4]
```

Then pass it to RESCU and execute the program as follows

```
rescu -i si_lcao_scf.txt;
```

The calculation is significantly faster. This is because the Kohn-Sham eigenvalue problem is projected onto the (small) AO subspace. Note that forces are not calculated at the end of atomic orbital calculations as Pulay contributions have not been implemented yet. Relaxation and phonon calculations cannot be performed using an AO basis consequently.