Developments in quantum dynamics of full-dimensional diatom-diatom collisions at pre-exascale*

Accurate rate coefficients for molecular rovibrational transitions due to collisions with H₂ are critical for interpreting IR astronomical observations. Theoretical results are the primary source of such rate coefficients. The most accurate theoretical approach is the quantum close-coupling method. Recently we extended full-dimensional quantum dynamics calculations of rovibrationally inelastic scattering large systems including CO-H₂, CN-H₂, SiO-H_2, and CS-H₂. The rovibrational cross sections have been computed using various implementations of the TwoBC code based on 6D potential energy surfaces. To date, full-D scattering calculations are mainly focused on the target molecule in its ground and first excited vibrational states with H₂ treated as a rigid rotor. To perform scattering computations with larger vibrational excitation of both diatoms further increases the computational demands. This relies on the availability of leadership-class computational resources. We present preliminary results for rovibrational scattering computations on Titan, SummitDev, and Summit with a progression
of optimization efforts with MPI, OpenMP, OpenACC, and Magma.