## `eigenkit.chemistry.interop`

OpenFermion interop (the optional `chemistry` extra).

OpenFermion's `FermionOperator` and `QubitOperator` share our term structure exactly — a `FermionOperator` term is a tuple of `(mode, action)` with `action` 1=creation / 0=annihilation, and a `QubitOperator` term is a tuple of `(qubit, 'X'|'Y'|'Z')` — so the conversions are direct. These adapters let you (a) bring an OpenFermion-built Hamiltonian onto the spine, and (b) reach mappings OpenFermion provides (e.g. Bravyi-Kitaev) via our `Observable`.

The `->` conversions read only the `.terms` mapping (no OpenFermion import needed); the `to_openfermion` direction imports OpenFermion lazily.

### `fermion_operator_from_openfermion(operator)`

Convert an `openfermion.FermionOperator` to an eigenkit `FermionOperator`.

Parameters:

| Name       | Type              | Description                        | Default    |
| ---------- | ----------------- | ---------------------------------- | ---------- |
| `operator` | `FermionOperator` | The fermionic operator to convert. | *required* |

Returns:

| Type              | Description                                        |
| ----------------- | -------------------------------------------------- |
| `FermionOperator` | The same operator, as an eigenkit FermionOperator. |

### `observable_from_openfermion(qubit_operator, *, atol=1e-10)`

Convert an `openfermion.QubitOperator` to an `(Observable, offset)` pair.

The identity term becomes the real `offset`. The coefficients must be real, because the operator must be Hermitian, and a `ValueError` is raised otherwise.

Parameters:

| Name             | Type            | Description                                                                  | Default    |
| ---------------- | --------------- | ---------------------------------------------------------------------------- | ---------- |
| `qubit_operator` | `QubitOperator` | The operator to convert.                                                     | *required* |
| `atol`           | `float`         | Terms whose coefficient is smaller than this value in magnitude are dropped. | `1e-10`    |

Returns:

| Name         | Type         | Description                              |
| ------------ | ------------ | ---------------------------------------- |
| `observable` | `Observable` | The operator, without its identity term. |
| `offset`     | `float`      | The coefficient of the identity term.    |

### `to_openfermion_fermion_operator(operator)`

Convert an eigenkit `FermionOperator` to an `openfermion.FermionOperator`.

Parameters:

| Name       | Type              | Description                        | Default    |
| ---------- | ----------------- | ---------------------------------- | ---------- |
| `operator` | `FermionOperator` | The fermionic operator to convert. | *required* |

Returns:

| Type              | Description                        |
| ----------------- | ---------------------------------- |
| `FermionOperator` | The same operator, in OpenFermion. |