# Tutorial¶

## Getting started with FermiLib¶

Installing FermiLib requires pip. Make sure that you are using an up-to-date version of it. Then, install FermiLib, by running

python -m pip install --pre --user fermilib

Alternatively, clone/download this repo (e.g., to your /home directory) and run

cd /home/fermilib
python -m pip install --pre --user .

This will install FermiLib and all its dependencies automatically. In particular, FermiLib requires ProjectQ . It might be useful to install ProjectQ separately before installing FermiLib as it might require setting some manual options such as, e.g., a C++ compiler. Please follow the ProjectQ installation instructions. FermiLib is compatible with both Python 2 and 3.

## Basic FermiLib example¶

To see a basic example with both fermionic and qubit operators as well as whether the installation worked, try to run the following code.

from fermilib.ops import FermionOperator, hermitian_conjugated
from fermilib.transforms import jordan_wigner, bravyi_kitaev
from fermilib.utils import eigenspectrum

# Initialize an operator.
fermion_operator = FermionOperator('2^ 0', 3.17)
fermion_operator += hermitian_conjugated(fermion_operator)
print(fermion_operator)

# Transform to qubits under the Jordan-Wigner transformation and print its spectrum.
jw_operator = jordan_wigner(fermion_operator)
jw_spectrum = eigenspectrum(jw_operator)
print(jw_operator)
print(jw_spectrum)

# Transform to qubits under the Bravyi-Kitaev transformation and print its spectrum.
bk_operator = bravyi_kitaev(fermion_operator)
bk_spectrum = eigenspectrum(bk_operator)
print(bk_operator)
print(bk_spectrum)

This code creates the fermionic operator $$a^\dagger_2 a_0$$ and adds its Hermitian conjugate $$a^\dagger_0 a_2$$ to it. It then maps the resulting fermionic operator to qubit operators using two transforms included in FermiLib, the Jordan-Wigner and Bravyi-Kitaev transforms. Despite the different representations, these operators are iso-spectral. The example also shows some of the intuitive string methods included in FermiLib.

Further examples can be found in the docs (Examples in the panel on the left) and in the FermiLib examples folder on GitHub.

## Plugins¶

In order to generate molecular hamiltonians in Gaussian basis sets and perform other complicated electronic structure calculations, one can install plugins. We currently support Psi4 (plugin here, recommended) and PySCF (plugin here).