Quantum Dynamics of van der Waals Molecular Clusters with MCTDH: Potential Energy Surface Representation, Rovibrational States Calculations, and Collisional Dynamics

Abstract

This research aims to advance the quantum dynamics simulation of van der Waals molec ular clusters, including H2O–H2O, H2O–HCN, and N2O–CO, which are formed with abundant molecules in the interstellar medium (ISM) such H2O and CO. While these sys tems have traditionally been studied through classical methods, the full quantum approach using the Multi-Configuration Time-Dependent Hartree (MCTDH) method was chosen to address challenges related to dimensionality. A critical first step in this approach is the accurate representation of the Potential Energy Surface (PES), as it underpins all subse quent calculations. Although previous studies have characterized some properties of these clusters, this work provides comprehensive rovibrational state calculations and initiates collision simulations for the water dimer. Achieving convergence, however, has proven challenging, as it is highly sensitive to the size of the primitive basis set and the num ber of single-particle functions (SPFs) utilized in flux calculations for computing inelastic cross sections and rate coefficients. Spectroscopic calculations were performed using the Block Improved Relaxation procedure from the Heidelberg MCTDH package, allowing ex traction of transition frequencies and rotational constants. The zero-point energy (ZPE) and intermolecular vibrational frequencies obtained from these simulations align well with prior ab initio studies, while the transition frequencies and rotational constants show good agreement with experimental data.