Hello there to the community, hope all are doing good.
I was performing the DMRG to find ground energy and energy gap between first excited state and ground state.
I got the energy gap of the order 10^ -11 which is very small
my question is, is this the excited state or the degenerate ground state.
so the function energy,psi1 , energy,psi2 are actually excited states or they can be degenerate ground and degenerate first excited states?
With thanks and regards.
To distinguish whether it is truly gapped or gapless, you would need to extrapolate the gap as a function of system size and bond dimension. Both finite system sizes and finite MPS bond dimensions will induce a nonzero gap, even if the exact state in the thermodynamic limit is gapless.