Neural Network-based Potential Energy Surfaces And Dynamics Of Classic Systems

Molecular reaction dynamics is at the heart of chemistry,which investigates the chemical process of the elementary reaction on molecular level and is of foundational importance in many fields,including combustion,atmospheric chemistry,interstellar chemistry,and plasma chemistry.Potential energy surface（PES）plays a key role in the research of chemical dynamics,because the dynamical results are due in large part to the accuracy of the PES.Thanks to the Born-Oppenheimer approximation,the PES can be represented as a function of nuclear coordinates.In the past decades,many researchers dedicated themselves to developing accurate fitting methods,the permutation invariant polynomial-neural network（PIP-NN）is one of the excellent fitting methods,which satisfies both the permutation symmetry and high accuracy.In this thesis,we develope the full-dimensional potential energy surfaces for the OH+HO₂,N₂O+C₂H₂,CO+H₂O,and C₂H₂+Ne systems,resepectively,using PIP-NN fitting method based on high level ab initio energy points.Then,comprehensive dynamical simulations are carried out to provide quantitative and full understanding for these systems.Measuring accurate rate coefficients of the OH+HO₂→H₂O+O₂reaction is of considerable theoretical and experimental interest due to its important role in atmosphere and combustion.The single-reference method coupled cluster referred to as the gold standard cannot provide a reliable description for the electronic structure of the radical-radical system due to its complex electronic structure and multi-reference character.Fortunately,in the OH+HO₂→H₂O+O₂reaction,the correct convergence to the ground state can be achieved by using appropriate Hartree-Fock initial guesses.The first full-dimensional triplet state PES of the OH+HO₂ system is constructed using the PIP-NN fitting method based on 108 000 points calculated at the CCSD（T）-F12a/AVTZ level.Then the quasi-classical trajectory（QCT）,ring-polymer molecular dynamics（RPMD）,and quantum dynamics（QD）methods are employed to investigate the rate coefficients of the reaction,respectively.The results are in good agreement with the previous theoretical and experimental measurements.In addition,in the study of mode specificity,we found vibrational excitation of the spectator O-H mode can significantly enhance the reactivity at low collision energy.This surprising enhancement of reactivity can be attributed to the increased dipole of the vibrationally excited OH,which strengthens the attraction between OH and HO₂.The deepening of the pre-TS well facilitates the capture at low collision energy,thus enhancing the reactivity.This work presents the first demonstration of vibrational control of chemical reactivity by exciting a spectator bond,enriching our understanding on mode-specific chemistry.The reaction between N₂O and C₂H₂ to form oxadiazole is a typical 1,3-dipolar cycloadditions.To have a deeper and reliable understanding of the chemical dynamics of this reaction,we develope the first full-dimensional PES of the reaction using PIP-NN fitting method based on 64 000 ab initio points at the level of CCSD（T）-F12a/AVDZ.Then,Comprehensive quasi-classical trajectory calculations are carried out to provide quantitative chemical insight into its mode specificity.In addition,we found the reaction takes place via a concerted mechanism by analyzing the time gap between the two forming bonds,which is also confirmed by the detailed statistical analysis on the geometries of the initial reactant and TS in reactive trajectories.H₂O and CO extensively exist in atmosphere,combustion of hydrocarbons,and the interstellar medium（ISM）,interaction and collision energy transfer between them is of considerable interest to scientist.In this study,the first full-dimensional PES of this reaction with the fitting error of only 0.025 kcal mol^-1 is constructed based on 102 000ponits calculated at the CCSD（T）-F12a/AVTZ level using PIP-NN fitting method.Based on the PIP-NN PES,the dependences of CO/OH on the OC-H₂O and CO-H₂O interaction energies are investigated.Then the quasi-classical trajectory calcualtions are employed to study the collision energy transfer dynamics between H₂O and CO at different initial conditions,including different initial vibrational energies of H₂O and different excitations for the CO stretch motion.The results illustrate that the total average energy transfer of H₂O is increased with the increase of the initial vibrational energy of H₂O and the excitation of CO can also enhance the total energy transfer of H₂O.The interaction potentials play a key role in the study of collison energy transfer,some empirical potential functions were usually adopted to describe the collisional energy transfer in the previous work.These simplified intermolecular PESs can introduce errors,in particular,for systems with significant anisotropic intermolecular interactions.In this study,we develope the full-dimensional intecation PES between Ne and C₂H₂using PIP-NN fitting method with basis set superposition error（BSSE）correction.Meanwhile,to comparison,the LJ 12-6 and Exp-6 potentials for the Ne+C₂H₂ sysytem are also used.Then,QCT simulations are performed to study the collision energy transfer dynamics on the three PESs,respectively.The results indicate that LJ-form interaction potential cannot produce reliable dynamical results,especially for rare events,such as highly efficient collisions.The Exp-6 PES works much better than the LJ 12-6 PES and the results calculated on Exp-6 PES are closed to the PIP-NN PES.