MOLSCAT: A program for non-reactive quantum scattering calculations on atomic and molecular collisions

Abstract

molscat is a general-purpose program for quantum-mechanical calculations on nonreactive atom– atom, atom–molecule and molecule–molecule collisions. It constructs the coupled-channel equations of atomic and molecular scattering theory, and solves them by propagating the wavefunction or log- derivative matrix outwards from short range to the asymptotic region at long range. It then applies scattering boundary conditions to extract the scattering matrix (S matrix). Built-in coupling cases include atom + rigid linear molecule, atom + vibrating diatom, atom + rigid symmetric top, atom + asymmetricorsphericaltop,rigiddiatom+rigiddiatom,rigiddiatom+asymmetrictop,anddiffractive scattering of an atom from a crystal surface. Interaction potentials may be specified either in program input (for simple cases) or with user-supplied routines. For the built-in coupling cases, molscat can loop over total angular momentum (partial wave) and total parity to calculate elastic and inelastic integral cross sections and spectroscopic line-shape cross sections. Post-processors are available to calculate differential cross sections, transport, relaxation and Senftleben–Beenakker cross sections, and tofittheparametersofscatteringresonances. molscat alsoprovidesaninterfaceforplug-inroutinesto specify coupling cases (Hamiltonians and basis sets) that are not built in; plug-in routines are supplied to handle collisions of a pair of alkali-metal atoms with hyperfine structure in an applied magnetic field. For low-energy scattering, molscat can calculate scattering lengths and effective ranges and can locateandcharacterisescatteringresonancesasafunctionofanexternalvariablesuchasthemagnetic field.