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

Code authors: Evangelista Lab
Documentation: Read the Docs

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:

conda install -c conda-forge forte2

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

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 .

You can run the standard fast test subset with:

pytest -m "not slow"

or run the full test suite with:

pytest

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:

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.MCSCF(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.

Forte2 interfaces with both the libint2 and libcint integral engines behind the scenes, and provides a common and simple interface for obtaining integrals in Python, as shown below.

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)

Forte2 is under active development, with an emphasis on multi-reference and relativistic methods. Current capabilities include (non-exhaustive, see also the Capabilities 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
• API Reference

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.