---
title: Filtering Functions (latest version)
description: API reference for Filtering Functions in the latest version of qiskit-fermions-c
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-fermions-c/qf-operators-terms-filtering
---

# Filtering Functions

### qf\_ferm\_op\_filter\_diagonal\_terms

`void qf_ferm_op_filter_diagonal_terms(QfFermionOperator *op)`

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

This removes every term from the provided [`QfFermionOperator`](/docs/api/qiskit-fermions-c/qf-ferm-op#qffermionoperator "QfFermionOperator") 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 [`qf_ferm_op_normal_ordered()`](/docs/api/qiskit-fermions-c/qf-ferm-op#qf_ferm_op_normal_ordered "qf_ferm_op_normal_ordered") for how to get an operator of that form.

```c
1QfFCIDump *fcidump = qf_fcidump_from_file("molecule.fcidump");
2QfFermionOperator *op = qf_ferm_op_from_fcidump(fcidump);
3
4QfFermionOperator *normal = qf_ferm_op_normal_ordered(op, NULL);
5
6qf_ferm_op_filter_diagonal_terms(normal);
```

> **Note**
>
> The operator is modified *in place*. If it tracks group indices (see [`qf_ferm_op_get_groups()`](/docs/api/qiskit-fermions-c/qf-ferm-op#qf_ferm_op_get_groups "qf_ferm_op_get_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.

**Parameters**

- **op** – A pointer to the normal-ordered fermionic operator whose terms are to be filtered (modified in place).