Neural network ensemble for computing cross sections of rotational transitions in H₂O + H₂O collisions

Water (H₂O) is one of the most abundant molecules in the universe and is found in a wide variety of astrophysical environments. Rotational transitions in H₂O + H₂O collisions are important for modeling environments rich in water molecules but they are computationally intractable using quantum mechanical methods. Here, we present a machine learning (ML) tool using an ensemble of neural networks (NNs) to predict cross sections to construct a database of rate coefficients for rotationally inelastic transitions in collisions of complex molecules such as water. The proposed methodology utilizes data computed with a mixed quantum-classical theory (MQCT). We illustrate that efficient ML models using NNs can be built to accurately interpolate in the space of 12 quantum numbers for rotational transitions in two asymmetric top molecules, spanning both initial and final states. We examine various architectures of data corresponding to each collision energy, symmetry of water molecules, and excitation/de-excitation rotational transitions, and optimize the training/validation data sets. Using only about 10% of the computed data for training, the NNs predict cross sections of state-to-state rotational transitions in H₂O + H₂O collisions with an average relative root mean squared error of 0.409. Thermally averaged cross sections, computed using the predicted state-to-state cross sections (∼90%) and the data used for training and validation (∼10%), were compared against those obtained entirely from MQCT calculations. The agreement is found to be excellent with an average percent deviation of about ∼13.5%. The methodology is robust, and thus applicable to other complex molecular systems.