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Draft: TD-DFT propagator enhancements by Pablo López Tarifa

Alberto Garcia requested to merge garalb/siesta:tarifa-propagators-mature into dev

This is a draft MR to keep the work by Pablo López Tarifa in the main development pipeline.

(The original branch is gitlab.com/lopeztarifa/siesta::propagators-mature. A MR from the original branch could be made)

Pablo said:

propagators-mature is a final version of the update in the schemes I did in the TDED.
The one we used in the Siesta JCP paper of 2020. The two-step scheme I implemented allows
a larger time step and better energy conservation for extreme cases like the inner shell
ionizations showcased in the paper.

However, what is more important of this branch is that Daniel (Sánchez-Portal) and I revised
the extrapolation formula (old Rafi’s evol1new subroutine) fixing the Hdiff term.
This change leads to better energy conservation in the extrapolation scheme.

For the latter reason, I think it is worth to be merged.

This MR is in principle orthogonal to !61 by Jessica Halliday. Emilio Artacho said:

The key contribution in Jessica’s is changing the way the movement of the basis is dealt with,
while Pablo’s is on the extrapolation of the Hamiltonian. They seem 
fairly independent, although there will be some interaction.

Authored-by: Pablo Lopez-Tarifa pablo.lopez.tarifa@gmail.com

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