Richmol is a Python package for simulations of rotational and rovibrational molecular spectra and dynamics in external fields.

The package is developed as part of ongoing research projects. Features are added based on project requirements and community interest.

Richmol currently supports:

• Rigid rotor dynamics in external fields, including dipole, polarizability, and hyperpolarizability interactions.
• Watson-type effective Hamiltonians (up to 6th-order).
• Absorption spectra involving dipole and quadrupole transitions, Raman transitions.
• Nuclear-spin hyperfine effects, with current support for quadrupole interactions.

The following capabilities are straightforward to add, depending on research needs and community contributions:

• Higher-order terms in Watson-type effective Hamiltonians.
• Vibrational effects using external matrix elements from programs like TROVE or vibrojet.
• Higher-order molecule-field interaction tensors, such as 2nd hyperpolarizability or interaction potentials expressed in terms of the Wigner $D$-matrices
• Spin-rotation and spin-spin hyperfine interactions.

Clone the repository and run

bash build_and_install.sh

Several examples can be found in the examples folder.

Documentation is a work in progress ...

If you use this code in your research, please cite:

A. Yachmenev, "Richmol: Python package for variational simulations of molecular nuclear motion dynamics in fields (2025)", available at: https://github.com/robochimps/richmol

@misc{Richmol,
  author = {Andrey Yachmenev},
  title  = {{Richmol}: {P}ython package for variational simulations of molecular nuclear motion dynamics in fields},
  year   = {2025},
  url    = {https://github.com/robochimps/richmol},
  note   = {available at: \url{https://github.com/robochimps/richmol}}
}

If you have questions regarding existing functionality or future requests, please open an issue or reach out to the authors directly at [email protected]