Fermionic Operators in SU2

[cfengno1]

Hi Huanchen,

In then get_npdm() defined in core.py:
if SymmetryTypes.SU2 in bw.symm_type: assert fermionic_ops is None
Why can't one use fermionic_ops in the SU2 symmetry? Can one use define fermionic_ops in the construction of MPO of the Hamiltonian?

Best,
Feng

[hczhai]

Can one use define fermionic_ops in the construction of MPO of the Hamiltonian?

When you set adjust_order=False when building the MPO, the value of fermionic_ops will not be used so it does not matter.

Why can't one use fermionic_ops in the SU2 symmetry?

This is related to the comment in #139 (reply in thread). In the SU2 mode, we do not allow custom operators, so that the block2 code knows which operators are fermionic and which are not. So fermionic_ops=None means it is not user's responsibility for defining the fermionicity in the SU2 mode.

In get_npdm(), the operator index for computing expectation values will inevitably have non-ascending ordering, so you can understand it as there is an enforced implicit adjust_order=True in get_npdm().