---
title: filter_diagonal_terms (latest version)
description: API reference for qiskit_fermions.operators.terms.filter_diagonal_terms in the latest version of qiskit-fermions
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-fermions/operators-terms-filter-diagonal-terms
---

# filter\_diagonal\_terms

`filter_diagonal_terms(op)`

Filters out the terms of an operator that are diagonal in the occupation-number basis.

This removes every term from the provided [`FermionOperator`](/docs/api/qiskit-fermions/operators-fermion-operator "qiskit_fermions.operators.FermionOperator") that is a product of number operators ($a^\dagger_i a_i$). This includes the constant term (a product of zero number operators), single number operators, as well as higher-order products such as $n_i n_j = a^\dagger_i a^\dagger_j a_j a_i$.

> **Hint**
>
> A major motivation for filtering such terms arises during the time evolution of operators, where such diagonal terms, do not affect the time evolution besides introducing a global phase. Consequently, bitstrings sampled from a time evolved state remain unaffected. Removing them is therefore recommended, for example when preparing an electronic-structure Hamiltonian for the qDRIFT time evolution used by SqDRIFT, as it avoids an otherwise unnecessary sampling overhead.

> **Caution**
>
> The provided operator *must* be normal-ordered! This is an underlying assumption of the implementation that is *not* being verified. See [`normal_ordered()`](/docs/api/qiskit-fermions/operators-fermion-operator#normal_ordered "qiskit_fermions.operators.FermionOperator.normal_ordered") for how to get an operator of that form.

> **Note**
>
> The operator is modified *in place*. If it tracks group indices (see [`groups`](/docs/api/qiskit-fermions/operators-fermion-operator#groups "qiskit_fermions.operators.FermionOperator.groups")), surviving terms retain their relative grouping but the group indices are reassigned to a contiguous range starting from 0. You must therefore *not* rely on the specific group index of any term being preserved across a call to this function.

```pycon
>>> from qiskit_fermions.operators import FermionOperator
>>> from qiskit_fermions.operators.terms.filtering import filter_diagonal_terms
>>> op = FermionOperator.from_dict(
...     {
...         (): 1.0,  # constant (diagonal)
...         ((True, 0), (False, 0)): 2.0,  # n_0 (diagonal)
...         ((True, 0), (True, 1), (False, 1), (False, 0)): 3.0,  # n_0 n_1 (diagonal)
...         ((True, 0), (False, 1)): 4.0,  # a†_0 a_1 (off-diagonal)
...     }
... )
>>> filter_diagonal_terms(op)
>>> list(op.iter_terms())
[([(True, 0), (False, 1)], (4+0j))]
```

**Parameters**

**op** – the normal-ordered operator whose diagonal terms to filter out.