TURBOMOLE Modules => Ridft, Rdgrad, Dscf, Grad => Topic started by: JakubV on April 18, 2021, 02:08:29 AM

Title: Localized Hartree-Fock / Localized DFT?
Post by: JakubV on April 18, 2021, 02:08:29 AM
Dear Turbomole experts,

I would like to have maybe a bit stupid question - the Localized HF as the name suggests, is HF being done in a localized basis set of molecular orbitals? Or not?

I suppose that if dscf/ridft would run in a localized MO basis one could be able to relatively quickly compute energies and other properties even for several hundred atom molecules/clusters of molecules...

Which is exactly what I want to do. I have a molecular complex of three molecules each 50 atoms large but surrounded with an environment of nine such 50 atoms (carbon, boron, hydrogen, few sulphurs, no heavy atom) molecules. I would like to have some PBE or B3LYP single-point or even geometry optimization.

I see that by traditional DFT/HF it is rather unrealistic even in minimal basis sets such as SV. MARIJK might help, but I bet - not so much. But localization would be a game-changer in speed-up.

So I will use lhfprep and lhf in the dft module somehow? Or is my point of view completely wrong, lhf in Turbomole means something completely different and my aim is impossible with Turbomole?

Best regards,
Title: Re: Localized Hartree-Fock / Localized DFT?
Post by: chris.hol on April 18, 2021, 09:26:01 AM
Dear JakubV,

the systems you are aiming for are perfectly fine for Turbomole. Roughly 600 mostly light atoms (C,H,N,O etc) with a few heavier atoms are perfectly fine for single-point energies, geometry optimizations and even for excited state TD-DFT calculations. I would no longer call them unrealistic as we run calculations of that size rather regularly - even using a larger def2-TZVP basis set : ).

For pure DFT functionals like PBE actually MARI-J is the game changer, and adding $marij to the control file will make such a calculation perfectly feasible within ridft/rdgrad for both single point energies and geometry optimizations. Pure DFT functionals have no need for HF exchange, and therefore neither localized HF nor any other exchange approximation is needed to run really quick. For functionals like B3LYP which include HF exchange, you may want to try $senex. This will trigger a seminumerical evaluation of exchange which is also pretty quick - actually much faster than localized HF.

Concerning localized HF (lhf), actually the aim of it is different. While the assumption that it localized HF exchange is correct, it was never built with maximum speed in mind. Instead, it aims at proving the Slater potential on a grid, i.e. a method to convert HF into an exchange-only DFT functional (okay, this is a pretty simple explanation - for more details I can send you links to the appropriate literature). This is probably not what you are looking for : ).

All the best,