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Standalone, open-source suite of quantum chemistry methods for strongly correlated electrons

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# Forte2

Forte2 is a standalone, open-source suite of quantum chemistry methods, with a focus on multi-reference and relativistic methods.

**Code authors**: [Evangelista Lab](https://evangelistalab.org/index.html)  
**Documentation**: [Read the Docs](http://forte2.readthedocs.io/en/latest/)

> [!WARNING]
> Forte2 is currently in pre-release development.
> Interfaces and behavior may change between releases.
> Please report bugs or breaking behavior in the [issue tracker](https://github.com/evangelistalab/forte2/issues).

## Installation

### Option 1: Install from conda-forge
Forte2 is released on conda-forge for Linux (x86_64, aarch64, and ppc64le) and macOS (x86_64 and arm64) platforms.
If your platform is supported, this is the easiest way to use Forte2. Simply run:
```bash
conda install -c conda-forge forte2
```

### Option 2: Build from source

Use this path if your platform is not covered by conda-forge packages, or if you want to contribute to Forte2.

```bash
git clone git@github.com:evangelistalab/forte2.git
cd forte2
conda env create -f environment.yml
conda activate forte2
pip install --no-build-isolation -ve .
```

### Testing the installation

You can run the standard fast test subset with:

```bash
pytest -m "not slow"
```

or run the full test suite with:

```bash
pytest
```

## Quickstart

Forte2 adheres to a functional composition workflow, where users build a final object by applying a sequence of methods to a `System` object. For example, a typical workflow might look like:

```python
import forte2

system = forte2.System(
    xyz="""
    N 0.0 0.0 0.0
    N 0.0 0.0 1.2
    """,
    basis_set="cc-pvdz",
    auxiliary_basis_set="cc-pVTZ-JKFIT",
)

rhf = forte2.RHF(charge=0, econv=1e-12)(system)
avas = forte2.AVAS(
    selection_method="separate",
    subspace=["N(2p)"],
    num_active_docc=3,
    num_active_uocc=3,
)(rhf)
ci_solver = forte2.CISolver(
    states=[forte2.State(nel=14, multiplicity=1, ms=0.0),
            forte2.State(nel=14, multiplicity=3, ms=0.0)],
)
mcscf = forte2.MCOptimizer(ci_solver=ci_solver)(avas)

pt = forte2.DSRG_MRPT2(s=0.5)(mcscf)

pt.run()
```
This sets up a state-averaged driven similarity renormalization group second-order multi-reference perturbation theory (DSRG-MRPT2) calculation on the nitrogen molecule, using the cc-pvDZ basis set and the cc-pVTZ-JKFIT auxiliary basis set, with an initial active space most resembling the 6 nitrogen 2p orbitals, automatically selected by the AVAS procedure.

## Obtaining integrals

Forte2 interfaces with both the [libint2](https://github.com/evaleev/libint) and [libcint](https://github.com/sunqm/libcint) integral engines behind the scenes, and provides a common and simple interface for obtaining integrals in Python, as shown below.

```python
import forte2

system = forte2.System(
    xyz="C 0 0 0; N 0 0 1.4",
    basis_set="cc-pvdz",
    auxiliary_basis_set="cc-pvdz-jkfit",
)

S = forte2.integrals.overlap(system)
T = forte2.integrals.kinetic(system)
V = forte2.integrals.nuclear(system)
# the order is (P|mn) in row-major order regardless of backend
# P is the auxiliary basis and m, n are the orbital basis
B = forte2.integrals.coulomb_3c(system)
```

## Capabilities

Forte2 is under active development, with an emphasis on multi-reference and relativistic methods.
Current capabilities include (non-exhaustive, see also the [Capabilities](https://forte2.readthedocs.io/en/latest/guide/capabilities.html) section of the user guide for more details):

- All methods use density-fitted or Cholesky-decomposed two-electron integrals
- Non-relativistic and relativistic SCF (RHF, ROHF, UHF, CUHF, GHF)
- CI methods including spin-adapted CI, GAS/ORMAS, and relativistic CI
- Heat-bath CI (HCI)
- MCSCF methods (CAS/GAS/ORMAS), with state averaging, and relativistic variants thereof
- AVAS active-space selection (one- and two-component workflows)
- DSRG-MRPT2 (non-relativistic and relativistic)
- Orbital analysis/manipulation tools (ASET, IAO, IBO, cube generation)

For detailed method documentation and theory references, see:
- [User Guide](http://forte2.readthedocs.io/en/latest/guide/index.html)
- [API Reference](http://forte2.readthedocs.io/en/latest/modules.html)

## Contributing

We welcome contributions of all kinds, including bug reports, feature requests, documentation improvements, and code contributions.

For code contributions, see notes on coding style and docstring standards in the [Contributor Guide](http://forte2.readthedocs.io/en/latest/contributor.html).

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