Use removeQNs to partially remove QNs for MPS in Anderson-Holstein Model

Question

Hi there,

I wanna run tDMRG for Anderson-Holstein model.

Because the initial state is with zero boson, at the step of DMRG, I restrict "ConserveNf=true, ConserveSz=true, ConserveNb=true", and set initial boson number to 0, so that I can find the ground state of the decouped boson-fermion system easily.

The MPS I got from DMRG step are with all those QNs for fermion and Boson. But after the quench, the boson number is no longer conserved, and will grow, so I need to remove the conservation number associated with boson. But I still want to keep the conservation related to fermion, "ConserveNf=true, ConserveSz=true".

I know I should use removeQNs to remove QNs from MPS, but how should I remove QNs partially?

Thank you very much!

Answer 1

Feng,
This is a great idea for a feature we should have, so I opened an issue for it on our issue tracker page on Github.

However, unfortunately we will not be able to add this feature soon, as once we thought about the technical steps they are somewhat involved and we are also working to complete the Julia version of ITensor.

As a short-term fix, would you be able to just not conserve boson number in your initial ground state calculation? Maybe the ground state of that Hamiltonian has zero boson number anyway? Or if not, maybe you can include a chemical potential term to tune to the zero-boson-number ground state case?

You can always measure the total expected boson number after DMRG of course, to check.

Best regards,
Miles