# Operator Library Building on top of the operator representations provided by [`operators`](qiskit_fermions.operators.html.md#module-qiskit_fermions.operators), this module provides various tools to simplify the construction of commonly used operators. ## Commutator Generators Operator classes can implement the [`SupportsCommutators`](../stubs/qiskit_fermions.protocols.SupportsCommutators.html.md#qiskit_fermions.protocols.SupportsCommutators) [`Protocol`](https://docs.python.org/3/library/typing.html#typing.Protocol), which allows them to be used with the functions listed in the table below, to quickly compute multiple commutator variants. ## Common Operators Various common operators can easily be generated from constructor methods. This section provides an overview of these methods for a quick reference, grouped by category. ### Electronic Integrals The constructor methods listed here generally take the coefficients of electronic structure Hamiltonians as an input. Different flavors exist: * `tril`: these methods consume 1-dimensional arrays of flattened (generalized) triangular indices * `full`: these methods consume high-dimensional arrays * `spin`: these methods take separate arrays for the different spin species * `sym`: these methods take a single array for one spin species and infer the other spin species The same `tril` and `full` vocabulary names the layout of arrays that are *returned* rather than consumed, such as the integral accessors of [`FCIDump`](../stubs/qiskit_fermions.operators.library.FCIDump.html.md#qiskit_fermions.operators.library.FCIDump). * 1-Body Terms | [`FermionOperator.from_1body_tril_spin_sym()`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator.from_1body_tril_spin_sym) | Constructs from spin-symmetric triangular 1-body integrals. | |------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------| | [`FermionOperator.from_1body_tril_spin()`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator.from_1body_tril_spin) | Constructs from separate spin triangular 1-body integrals. | * 2-Body Terms | [`FermionOperator.from_2body_tril_spin_sym()`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator.from_2body_tril_spin_sym) | Constructs from spin-symmetric triangular 2-body integrals. | |------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------| | [`FermionOperator.from_2body_tril_spin()`](../stubs/qiskit_fermions.operators.FermionOperator.html.md#qiskit_fermions.operators.FermionOperator.from_2body_tril_spin) | Constructs from separate spin triangular 2-body integrals. | ## Other Generators Finally, the following additional operator generator utilities exist in this module: