User Manual for MOLSCAT, BOUND and FIELD, Version 2025.0: programs for quantum scattering properties and bound states of interacting pairs of atoms and molecules

TL;DR

MOLSCAT, BOUND, and FIELD Version 2025.0 provide a unified, general-purpose suite for non-reactive quantum scattering and bound-state calculations of atom–atom and atom–molecule systems using a robust coupled-channel framework. The theory section details the Hamiltonian, the coupled-channel formalism, and the expansion of the interaction potential, with explicit treatment of both diagonal and non-diagonal internal/centrifugal matrices and the transformation to asymptotic bases. The manual comprehensively documents input formats, basis construction, potential expansions, propagators, external-field handling, and the full set of outputs, including S/K matrices, cross sections, line-shape calculations, and bound-state energies, along with automated convergence and resonance-finding capabilities. Significant updates in 2019–2025 add flexible plug-in basis-set suites, improved external-field treatment, harmonic confinement, modernized constants and memory management, and extensive documentation and example coverage to support a wide range of ultracold and collision phenomena and resonant processes in chemical physics.

Abstract

MOLSCAT is a general-purpose package for performing non-reactive quantum scattering calculations for atomic and molecular collisions using coupled-channel methods. Simple atom-molecule and molecule-molecule collision types are coded internally and additional ones may be handled with plug-in routines. Plug-in routines may include external magnetic, electric or photon fields (and combinations of them). Simple interaction potentials are coded internally and more complicated ones may be handled with plug-in routines. BOUND is a general-purpose package for performing calculations of bound-state energies in weakly bound atomic and molecular systems using coupled-channel methods. It solves the same sets of coupled equations as MOLSCAT, and can use the same plug-in routines if desired, but with different boundary conditions. FIELD is a development of BOUND that locates external fields at which a bound state exists with a specified energy. One important use is to locate the positions of magnetically tunable Feshbach resonance positions in ultracold collisions. The program source code is available from https://github.com/molscat/molscat