Page 1 of 1

Trying to fit DDH parameters

Posted: Wed Jul 17, 2024 8:17 am
by jake_burner1
Hi there,

I would like to run some TD-DDH calculations, but as a first step want to fit the DDH parameters to my materials using the dielectric function (as was done in this paper DOI: 10.1103/PhysRevMaterials.2.073803). Is the only method to compute the dielectric function via RPA, or can I get this function from just running a calculation with LOPTICS=.TRUE.? My understanding is I need to plot the dielectric constant as a function of G, but the output of LOPTICS=.TRUE. gives me the dielectric as a function of energy. Perhaps there is something obvious I am overlooking, or I have a big misunderstanding. This is not exactly my area of expertise, but I guess I am wondering if I can get an approximation of these parameters without running an expensive RPA calculation, as was done in the TD-DDH tutorial posted on the VASP wiki.

Thanks for any help or guidance!
Jake

Re: Trying to fit DDH parameters

Posted: Wed Jul 17, 2024 9:26 am
by pedro_melo
Dear Jake,

The answer to your question depends on the approximation to the dielectric function that you want to take. If you use LOPTICS=.TRUE. during a ground-state calculation with extra, empty states, you will obtain the independent-particle macroscopic dielectric function, which is independent of G and has no electron-hole interaction taken into account. To perform a TD-DDH calculation you will need to provide VASP with \epsilon^{-1}_\infty and \mu parameters in order to model the G-dependent dielectric function (see LMODELHF for more).

Keep in mind that the model only includes the diagonal components of the dielectric function, which is normally a tensor in (G,G'). So there are no elements with G different from G'.

In our page for LMODELHF we provide some information of the value to use for a list of materials. Of course, if you have a reference value of those parameters for the material you want to study you can just use them in the INCAR.

Kind regards,
Pedro

Re: Trying to fit DDH parameters

Posted: Wed Jul 17, 2024 10:25 am
by jake_burner1
Dear Pedro,

Thank you for your detailed reply, this has helped me to identify some gaps in my understanding. I have also come across some methods of parameterizing these functional parameters using DFPT to obtain the macroscopic dielectric constant (and thereby determine AEXX), and using an effective Thomas-Fermi screening parameter to approximate \mu (HFSCREEN) (DOI: 10.1038/s41524-022-00869-6). I guess this bypasses the requirement for an RPA calculation (which I understand now is necessary to parameterize according to the methodology in the previous paper I posted), but presents other issues, particularly for small-gap semiconductors.

Have a nice day!
Jake