MOLSCAT: A program for nonreactive quantum scattering calculations on atomic and molecular collisions
Abstract
MOLSCAT is a generalpurpose program for quantummechanical calculations on nonreactive atomatom, atommolecule and moleculemolecule collisions. It constructs the coupledchannel equations of atomic and molecular scattering theory, and solves them by propagating the wavefunction or logderivative 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). Builtin coupling cases include atom + rigid linear molecule, atom + vibrating diatom, atom + rigid symmetric top, atom + asymmetric or spherical top, rigid diatom + rigid diatom, rigid diatom + asymmetric top, and diffractive scattering of an atom from a crystal surface. Interaction potentials may be specified either in program input (for simple cases) or with usersupplied routines. For the builtin coupling cases, MOLSCAT can loop over total angular momentum (partial wave) and total parity to calculate elastic and inelastic integral cross sections and spectroscopic lineshape cross sections. Postprocessors are available to calculate differential cross sections, transport, relaxation and SenftlebenBeenakker cross sections, and to fit the parameters of scattering resonances. MOLSCAT also provides an interface for plugin routines to specify coupling cases (Hamiltonians and basis sets) that are not built in; plugin routines are supplied to handle collisions of a pair of alkalimetal atoms with hyperfine structure in an applied magnetic field. For lowenergy scattering, MOLSCAT can calculate scattering lengths and effective ranges and can locate and characterise scattering resonances as a function of an external variable such as the magnetic field.
Program Files doi:http://dx.doi.org/10.17632/rtzgf5mwpn.1
Licensing provisions: GPLv3
Programming language: Fortran 90
External routines/libraries: LAPACK, BLAS
Nature of problem: Quantummechanical calculations of scattering properties for nonreactive collisions between atoms and molecules.
Solution method: The Schrödinger equation is expressed in terms of coupled equations in the interparticle distance, R. Solutions of the coupledchannel equations are propagated outwards from the classically forbidden region at short range to the asymptotic region. The program calculates scattering S matrices and uses them to calculate scattering properties including elastic, inelastic and lineshape cross sections.
Unusual features:
1. MOLSCAT contains numerous features to handle quantities important in lowenergy collisions. It can propagate very efficiently to very long range, and it can calculate lowenergy properties such as the scattering length (complex and energydependent if required) and the effective range.
2. MOLSCAT can construct and solve sets of coupled equations using basis sets that are not eigenfunctions of the Hamiltonians of the colliding particles at long range. The propagation is carried out in the primitive basis set, and the solutions are transformed to the asymptotic basis set before applying longrange boundary conditions to extract the S matrix.
3. MOLSCAT provides an interface that allows users to specify the coupled equations that arise from additional Hamiltonians and basis sets.
4. MOLSCAT provides an interface that allows users to include multiple external fields in the Hamiltonian. This same interface also allows users to include a factor which scales the interaction potential, and to investigate how properties vary with this factor.
5. MOLSCAT can locate and characterise lowenergy Feshbach resonances either as a function of collision energy or as a function of external fields or of the potential scaling factor.
 Publication:

Computer Physics Communications
 Pub Date:
 August 2019
 DOI:
 10.1016/j.cpc.2019.02.014
 arXiv:
 arXiv:1811.09584
 Bibcode:
 2019CoPhC.241....9H
 Keywords:

 S matrix;
 Cross section;
 Elastic;
 Inelastic;
 Feshbach resonance;
 External fields;
 Physics  Chemical Physics;
 Condensed Matter  Quantum Gases;
 Physics  Atomic Physics
 EPrint:
 arXiv admin note: substantial text overlap with arXiv:1811.09111 This is because these are two parallel papers that describe related programs with the same theoretical basis but for different purposes