MQCT 2026: a program for calculations of inelastic scattering of two molecules (new version announcement)

This is a revised and updated version of the package MQCT [Comput. Phys. Commun. 252 (2020) 107155; 294 (2024) 108938]. The package includes routines for the calculations of ro-vibrational state-to-state transition cross sections for molecule + molecule collisions, using the mixed quantum/classical theory (MQCT) approach. One major improvement in computational efficiency is implemented for calculations of potential coupling matrix elements, which accelerates this part of calculations by several orders of magnitude. Another important addition to the package is an efficient method that combines the coupled-states (CS) approximation with the adiabatic-trajectory (AT) approach, named AT-CS-MQCT, which enables calculations for larger and more complex molecules at higher collision energies. Corrections to the treatment of indistinguishable collision partners, such as CO + CO and H2O + H2O, include proper incorporation of exchange parity and statistical weight factors. A revised version of user manual is provided.