Meanfield calculations: tricks and hints
Tricks and hints
Vxc0
Vxc0
is the G=0 component of the exchangecorrelation potential V_{xc}.
Vxc0
is added to the eigenvalues in the wavefunction files produced
by PARATEC and ESPRESSO. Vxc0
is also included in the VXC
and vxc.dat
files produced by PARATEC and ESPRESSO (the VXC
file contains Vxc(G)
and the vxc.dat
file contains matrix elements of V_{xc}). The two Vxc0
terms cancel out when calculating the quasiparticle corrections
in the Sigma code.
Vacuum level
To correct the DFT eigenvalues for the vacuum level take the average
of the electrostatic potential on the faces of the unit cell in the
nonperiodic directions and subtract it from the DFT eigenvalues.
The electrostatic potential is defined as V_{elec} = V_{bare} + V_{hartree},
while the total potential is V_{tot} = V_{bare} + V_{hartree} + V_{xc}, hence
V_{tot} contains Vxc0
and V_{elec} does not. The average of V_{elec} is
fed into the Sigma code using keywords avgpot
and avgpot_outer
.
The potentials can be generated with PARATEC and ESPRESSO.
In PARATEC, use elecplot
for V_{elec} or potplot
for V_{tot}, then
convert from a3dr
to cube using the Visual/volume.py
script.
In ESPRESSO, use iflag=3
, output_format=6
, and plot_num=11
for V_{elec} or plot_num=1
for V_{tot}. The averaging is done with
the Visual/average.py
script.
Unit cell size
If you truncate the Coulomb interaction, make sure that the size
of the unit cell in nonperiodic directions is at least two times
larger than the size of the charge distribution. This is needed
to avoid spurious interactions between periodic replicas but at
the same time not to alter interactions within the same unit cell.
Run Visual/surface.x
to plot an isosurface that contains 99% of the
charge density (see Visual/README for instructions on how to do this).
The code will print the size of the box that contains the isosurface
to stdout. Multiply the box dimensions in nonperiodic directions
by two to get the minimum size of the unit cell.
Inversion symmetry
When using the real flavor of the code, make sure the inversion
symmetry has no associated fractional translation (if it does,
shift the coordinate origin). Otherwise WFN
, VXC
, RHO
(and VSC
in SAPO) have nonvanishing imaginary parts which are simply
dropped in the real flavor of the code. This won't do any
good to your calculation.
Pitfalls
There are some notorious cases where typical DFT calculations might not provide a good starting point for oneshot perturbationtheory calculations. Examples include:

Germanium crystal, which is often predicted to be metallic at DFT. This issue can be remedied by either using another meanfield starting point, or performing some sort of selfconsistent iteration, for instance, based on the static COHSEX approximation.

Molecules such as silane (SiH4), in which semilocal DFT often yields a LUMO orbital that bound, where in reality it is unbound. These systems can also be remedied by using a different starting meanfield point, or performing selfconsistent GW calculations. For molecules, another common approach is known as best G, best W, where one picks one meanfield starting point to write the Green's function G in \Sigma=iGW, such as HartreeFock, and another one to compute the polarizability \chi^0 used to construct W, such as LDA.

Some strongly correlated systems, especially those with partially filled f orbitals. Systems such as transitionmetal oxides are often tackled with a Hubbardtype of correction scheme, such as DFT+U.