Lkind check allows for irreducible exciton calculation

[palful]

As discussed with Alberto, currently a check at line 166 of K_driver_init would not accept Lkind='tilde' as input option for irreducible exciton calculations, although this case is coded below at line 266 of the same file and also documented at input generation via INIT_load.

The change simply recognizes Lkind='tilde' as an option to switch off the exchange part of the BSE kernel.

Also note that

Lkind='tilde'

is equivalent - but more straightforward - to

Lkind='bar'
BSENGexx = 0 RL

[andrea-ferretti]

fine with me to proceed

[andrea-ferretti]

fine with me

[sangallidavide]

Just one comment on this point.

Instead of having to introduce a new "Lkind", I think it would be more straightforward to define the kernel approximations
BSKmod="Hartree+SEX" vs BSKmod="SEX"

The main drawback of this alternative is that, so far BSKmod="SEX" meant BSKmod="Hartree+SEX"

[palful]

I agree it is largely a matter of preference. For me, I'd rather let the user focus on the fact that they are computing different response functions:

• Lbar is the response to the total macroscopic fields
• Lfull is the response to the external field
• Ltilde is the response to the total (external + induced) fields

The BSKmod string instead goes more towards conceptualizing these as different approximations to an electron-hole kernel in transition space. I think it's fine as well.

The only important thing is that this information needs to be reported in a database (such as ndb.BS_diago: now it is written under a variable named X_kind) because yambopy needs it for some checks, such as when computing exciton symmetries at q=0, and in the future I plan to introduce more checks to better track the kinds of response functions used in the exciton-phonon coupling matrix elements (so the user doesn't get confused if they are computing the coupling with Lfull, Ltilde, etc).