# Long-Range Fit: A Software Package for the Representation and Study of Long-Range Molecular Interactions

## TL;DR

Long-range-fit is a software package that automates the study of long-range molecular interactions, making it easier to model molecular systems.

## Contribution

The software provides an automated, high-order, and symmetry-adapted method for generating long-range interaction terms.

## Key findings

- All terms up to 15th order were derived and implemented without approximations.
- The software supports symmetry adaptation for all molecular point-group symmetries.
- The potential energy surface is compatible with various representations of close interactions.

## Abstract

Describing intermolecular forces is fundamental to modeling
and
predicting the behavior of molecular systems. In particular, long-range
molecular interactionswith electrostatic, induction, and dispersion
as the main componentsplay a critical role, especially for
low-temperature and low-density regimes. Long-range interactions are
often described through perturbation theory, representing the electronic
charge distribution via a multipolar series of the moments and polarizability
tensors corresponding to each molecule. However, while the theory
is well established, obtaining the resulting analytical expressions
(and their practical implementation) constitutes a highly complex
and system-dependent task. To address this challenge, we developed
long-range-fit (LRF), an interactive and user-friendly software package
designed to automate the generation and fitting of long-range interaction
terms for arbitrary molecules in nondegenerate (ground or excited)
electronic states. We have derived and implemented all terms up to
15th order, without approximations, via a spherical tensor representation,
with symmetry adaptation to all molecular point-group symmetries.
The resulting potential energy surface is compatible with most representations
of the close interaction region.