# Kernel code input keywords (kernel.inp)

## Keyword documentation

### Screening type

How does the screening of the system behaves? (default=screening_semiconductor) BerkeleyGW uses this information to apply a different numerical procedure to computing the diverging $q\rightarrow 0$ contribution to the screened Coulomb potential $W_{GG'}(q)$. These models are not used in Hartree-Fock calculations.

#### screening_semiconductor

Use this flag on gapped system (default).

#### screening_metal

Use this flag on metallic system, i.e., constant DOS at the Fermi energy.

#### screening_graphene

Use this flag on systems with vanishing linear DOS at the Fermi level, such as graphene.

### Truncation schemes for the Coulomb potential

Since BerkerleyGW is a plane-wave-based code, one must truncate the Coulomb potential to avoid spurious interactions between repeated supercells when dealing with systems with reduced dimensionality. Make sure you understand how to setup your mean-field calculation so that the supercell is large enough to perform a truncation of the Coulomb potential.

#### cell_box_truncation

Truncate the Coulomb potential based on the Wigner-Seitz cell. This is the recommended truncation for 0D systems.

#### cell_wire_truncation

Truncation scheme for 1D systems, such as carbon nanotubes. The $z$ direction is assumed to be periodic, and $x$ and $y$ confined.

#### cell_slab_truncation

Truncation scheme for 2D systems, such as graphene or monolayer MoS2. The $z$ direction is assumed to be confined, and $x$ and $y$ periodic.

#### spherical_truncation

Truncate the Coulomb potential based on an analytical scheme. This is ok for quasi-spherical systems, such as CH4 molecule or C60, but the cell_box_truncation is the most general and recommended scheme. When using spherical truncation, you must also specify the radius for the truncation in spherical_truncation.

#### coulomb_truncation_radius [float]

This specifies the radius of for spherical truncation, in Bohr, so that the Coulomb potential $v(r)$ is zero for $r$ larger than these values. This flag is to be used together with spherical_truncation.

### Misc. parameters

#### number_val_bands [integer]

Number of valence bands to include in the kernel of the BSE, counting down from the highest occupied state.

#### number_cond_bands [integer]

Number of conduction bands to include in the kernel of the BSE, counting up from lowest unnocupied state.

#### screened_coulomb_cutoff [float]

Energy cutoff for the screened Coulomb interaction, in Ry. The screened Coulomb interaction $W_{GG'}(q)$ will contain all G-vectors with kinetic energy $|q+G|^2$ up to this cutoff. Default is the epsilon_cutoff used in the epsilon code. This value cannot be larger then the epsilon_cutoff or the bare_coulomb_cutoff.

#### bare_coulomb_cutoff [float]

Energy cutoff for the bare Coulomb interaction, in Ry. The bare Coulomb interaction $v(G+q)$ will contain all G-vectors with kinetic energy $|q+G|^2$ up to this cutoff. Default is the WFN cutoff.

#### fermi_level [float]

Specify the Fermi level (in eV), if you want implicit doping Note that value refers to energies after scissor shift or eqp corrections. See also fermi_level_absolute and fermi_level_relative to control the meaning of the Fermi level.

The Fermi level in keyword fermi_level can be treated as an absolute value or relative to that found from the mean field (default)

#### dont_use_hdf5

Read from traditional simple binary format for epsmat/eps0mat instead of HDF5 file format. Relevant only if code is compiled with HDF5 support.

#### verbosity [integer]

Verbosity level, options are:

• 1: default
• 2: medium - info about k-points, symmetries, and eqp corrections.
• 3: high - full dump of the reduced and unfolded k-points.
• 4: log - log of various function calls. Use to debug code.
• 5: debug - extra debug statements. Use to debug code.
• 6: max - only use if instructed to, severe performance downgrade. Note that verbosity levels are cumulative. Most users will want to stick with level 1 and, at most, level 3. Only use level 4+ if debugging the code.

These flags define whether to use symmetries to unfold the Brillouin zone or not in file WFN_co (use_symmetries_coarse_grid, no_symmetries_coarse_grid).

In most cases, you probably want to use symmetries to unfold the grid in WFN_co.

Warning

Default is not to unfold!

#### write_vcoul

Write the bare Coulomb potential $v(q+G)$ to file

#### low_comm

Low communication The default behavior of the code is to distribute the dielectric matrix among the processors. While this minimizes memory usage, it also increases the communication. By using the low_comm flag, each processor will store the whole dielectric matrix. It is advisable to use this flag whenever each PE has enough memory to hold the whole epsmat file.

#### low_memory

Low Memory option Calculate matrix elements separately for each k,c,v,k',c',v' pair

#### spinor

Request calculation of the Kernel using spinor wavefunctions

#### high_memory

High Memory option Save all wavefunction FFTs when calculating the BSE kernel. Overwrites the low_memory option.

#### fullbz_replace

'unfolded BZ' is from the kpoints in the WFN file 'full BZ' is generated from the kgrid parameters in the WFN file See comments in Common/checkbz.f90 for more details Replace unfolded BZ with full BZ

#### fullbz_write

Write unfolded BZ and full BZ to files

#### die_outside_sphere

Rotation of the k-points may bring G-vectors outside of the sphere. Use the following keywords to specify whether to die if some of the G-vectors fall outside of the sphere. The default is to ignore. Set to die in case screened_coulomb_cutoff = bare_coulomb_cutoff. Set to ignore in case screened_coulomb_cutoff < bare_coulomb_cutoff.

#### read_kpoints

Flag to read k-points from the 'kpoints' file. The default is to read k-points from the wfn file.

#### extended_kernel

Calculates the kernel for all $(n_1,n_2)\rightarrow(n_1',n_2')$ transitions, and not only the pairs involving $(v,c)\rightarrow(v'c')$ transitions necessary within by the Tamm-Dancoff approximation (TDA).

Notes:

1. With this option BerkeleyGW will calculate transitions beyond those needed by the full BSE, such as $(v,v)\rightarrow(c,c)$.
2. In principle, one should always calculate these extra kernel blocks, because the interpolation routine in absorption mixes wavefunction characters, in particular for metallic and narrow-gap semiconductors.
3. You do not need these extended kernel blocks only if you are not interpolating later on. However, the only way to do non-TDA absorption calculation right now is with the extended kernel flag, i.e., there's no intermediate option to include only $(v,c)\rightarrow(c'v')$ transitions but not $(v,v)\rightarrow(c'c')$.
4. Only parallelization such that number of processors $\le$ $k^2$ is supported.