Some effective Hamiltonians have two-body coefficients with only 4-fold permutation symmetry. This applies to the SW-PT2 implementation in #619 and may also apply to DUCC and CT-F12.
Such effective Hamiltonians preserve Hermiticity and electron exchange:
$$
(pq|rs) = (qp|sr), \qquad (pq|rs) = (rs|pq),
$$
but do not generally preserve the Coulomb bra-swap symmetry:
$$
(pq|rs) = (qp|rs).
$$
Two existing consumers currently assume full 8-fold ERI symmetry:
- The native
"qdk" qubit mapper uses 8-fold symmetry in its restricted path.
Hamiltonian.to_fcidump_file() writes only the canonical 8-fold-unique entries.
Passing a 4-fold-symmetric tensor to either path can therefore silently produce a different Hamiltonian, resulting in incorrect mapped energies or lossy FCIDUMP output.
These consumers should correctly handle Hamiltonians with reduced two-body permutation symmetry or reject unsupported inputs explicitly. Existing behavior for conventional 8-fold-symmetric electron-repulsion tensors should remain unchanged.
This should be addressed before merging #619.
Some effective Hamiltonians have two-body coefficients with only 4-fold permutation symmetry. This applies to the SW-PT2 implementation in #619 and may also apply to DUCC and CT-F12.
Such effective Hamiltonians preserve Hermiticity and electron exchange:
but do not generally preserve the Coulomb bra-swap symmetry:
Two existing consumers currently assume full 8-fold ERI symmetry:
"qdk"qubit mapper uses 8-fold symmetry in its restricted path.Hamiltonian.to_fcidump_file()writes only the canonical 8-fold-unique entries.Passing a 4-fold-symmetric tensor to either path can therefore silently produce a different Hamiltonian, resulting in incorrect mapped energies or lossy FCIDUMP output.
These consumers should correctly handle Hamiltonians with reduced two-body permutation symmetry or reject unsupported inputs explicitly. Existing behavior for conventional 8-fold-symmetric electron-repulsion tensors should remain unchanged.
This should be addressed before merging #619.