Hi Zhen,
I could see how this would be a tough system to study with DMRG. Fortunately you have done all the right things so far and thanks for studying the FAQ about preventing DMRG from getting stuck.
I think the main reason this system is hard to converge is that there is no hopping term between the f electrons. That’s just the nature of this system and the way it is defined, of course, but it can be helpful to start there.
Here are some things you could try, in the order that I would try them.
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the first is to also pass the parameter
eigsolve_krylovdimto thedmrgfunction and set the value of this parameter to something like 8 or 10 (the default is 3). It can sometimes make a big difference to DMRG on harder problems to set this to a higher value. -
the second idea is bit unusual, but could work well. This idea is to do two separate DMRG calculations, one following the other. In the first, you could define H to include a small nearest-neighbor hopping t' between the f electrons as well and converge DMRG for that system. Then take the MPS that is returned and use it to initialize a second DMRG calculation with the original Hamiltonian (now t'=0) and do some number of sweeps until that converges. If the first value of t' is small and all goes well, you may need very few sweeps of the second run of DMRG to get good results, since I believe the physics may be quite similar.
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if neither of the two above approaches help, there are some more things that can be tried but they might be more involved. One of them would be to do three-site DMRG rather than two-site, which can precisely help for problems like this where the unit cell along the MPS path is larger than two sites and when the Hamiltonian does not “naturally” mix degrees of freedom within a single unit cell (e.g. the f electrons only really mix with each other through their hybridization with the c electrons)
Hope that helps and please let me know how it goes. We can discuss more if some of the initial ideas don’t work –