13.3 General recipe for G0W0 calculations

The general recipe for a G0W0 calculation is as follows:

1.
define session
2.
Provide additional GW control flags
3.
dscf or ridft calculation
4.
escf calculation

Ad 1) Symmetry has not yet been implemented for GW, sensible calculations can only be done in C1. The def2-TZVPP basis seems to be the most useful, it comes for all tested systems within 0.1 eV of the def2-QZVP result with about half the number of basis functions. In the define session the calculation of the response function needs to be defined. In the final define menu select the ex menu and select the calculation of RPA singlet excitations, or urpa in case of open shell. Choose soghf to run a two-component calculation. Select "all all" to get all excitations. For systems and basis set having less than 4000 rpas excitations just set for all excitations. For large systems start to run G0W0 with ˜4000 rpas excitations. In subsequent runs add more excitations until a converged result is reached. escf will keep the converged roots, so not much time is lost using this restart approach.

Ad 2) If $gw is set in the control file the quasi particle energies will be evaluated according to equation 13.3. Additional options are described in the keyword section, section 20.2.14.

Ad 4) In the escf run the response function is calculated which is needed to determine the screened coulomb interaction. At the end of this run the actual GW calculation is performed.

Possible source of errors: When dscf or ridft is repeated after escf the sing_a file may not be correct anymore, this may happen when degenerate levels are present. escf will however not recognize this and continue using the previously converged data in sing_a leading to nonsense values for Σc. Before running escf the old sing_a file has to be removed.