Reaction Dynamics and Vibrational Studies of Atmospheric Species on Potential Energy Surfaces Open Access

Wang, Xiaohong (2016)

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The potential energy surface (PES) plays a signicant role in the theoretical studies of reaction dynamics and vibrational spectrums. In our work, the PES is obtained using weighted linear least-square fitting method with respect to tens of thousands of scattered electronic energies. The key feature of surfaces is that they are explicitly invariant under all permutations of the same nuclei, which is built into the polynomial basis used for the fitting. Lots of fundamental chemical reactions implicate crucial processes in atmospheric chemistry. Taking advantage of the above fitting techniques, we can perform detailed dynamics simulations of such reactions, including H2+CN, photo-dissociation of HOCO, unimolecular decay of Criegee Intermediate, CH3CHOO. The theoretical exploration, usually collaborating with the experimental studies, throws insight into the microscopic mechanism of the reactions. The availability of PESs also makes it practical to perform quantum vibrational studies of various systems. Several advanced quantum mechanical methods are implemented and applied in the dissertation work. Diusion Monte Carlo is applied to solve the ground vibrational states numerically exactly. The variational vibrational configuration interaction approach is used to obtain a large range of vibrational states simultaneously. We have performed the vibrational calculations of many atmospheric species, including C4, CH3CHOO and CH3NO2, which successfully support and guide the assignment of experimental observations. Besides, an efficient technique is implemented and tested in the latest vibrational calculations, which greatly reduces the computational expense without losing the accuracy.

Table of Contents

Chapter 1 Introduction. 1

I Theory and Methods. 3

Chapter 2 Potential Energy Surface and Dynamic Simulation. 4

2.1 Potential Energy Surface. 4

2.1.1 Born-Oppenheimer approximation. 6

2.1.2 Monomial Symmetrization. 7

2.1.3 Invariant Polynomial. 9

2.1.4 Dipole Moment Surface. 10

2.1.5 Procedure for Constructing PES. 11

2.2 Molecular Dynamic Simulation. 12

2.2.1 Normal Mode Sampling. 13

2.2.2 Rotation Sampling. 14

2.2.3 Zero Point Energy Constraint. 16

2.2.4 Final Conditions. 16

Chapter 3 Vibrational Calculation. 19

3.1 Diusion Monte Carlo. 20


3.2.1 Hamiltonian and Potential. 24

3.2.2 Variational Calculation. 25

3.2.3 Infrared Intensity. 27

II Application: Reaction Dynamics Simulation. 28

Chapter 4 Dynamic Simulations of H+HCN Reaction. 29

4.1 Introduction. 29

4.2 Potential Energy Surface Construction. 31

4.2.1 Ab Initio Calculation. 31

4.2.2 Potential Energy Surface Fitting. 32

4.2.3 Properties of PES. 34

4.3 Dynamic Simulations. 37

4.4 Summary and Conclusion. 49

4.5 PES Construction of Similar Systems. 50

Chapter 5 Unimolecular Dissociation of syn-CH3CHOO. 52

5.1 Introduction. 53

5.2 Potential Energy Surface. 54

5.3 Dynamic Studies in Experiment and Theory. 61

5.4 Summary. 75

5.5 Prompt Decay with High Internal Energy. 76

Chapter 6 Mode-specic tunneling of cis-HOCO Dissociation to H+CO2. 80

6.1 Introduction. 81

6.2 Method. 85

6.3 Results and Discussion. 90

6.4 Summary and Concluding Remarks. 97

III Application: Vibrational Calculation. 100

Chapter 7 Anharmonic rovibrational calculations of singlet cyclic C4 . 101

7.1 Introduction. 101

7.2 Computational Methods. 103

7.2.1 Quartic Force Field. 103

7.2.2 Semi-global PES and MULTIMODE Calculations. 105

7.3 Results. 107

7.4 Summary and Conclusions. 118

Chapter 8 Infrared Spectra of CH3CHOO. 120

8.1 Introduction. 120

8.2 Potential Energy Surface of CH3CHOO Isomers. 122

8.3 Diusion Monte Carlo. 128

8.4 Survey Spectrum of CH3CHOO. 130

8.5 Spectral Analysis of CH3CHOO in Fundamental Range. 135

8.6 Summary. 149

Chapter 9 Pruning the Hamiltonian Matrix in MULTIMODE. 150

9.1 Introduction. 150

9.2 Potential Energy Surface. 152

9.3 Diusion Monte Carlo. 156

9.4 MULTIMODE calculations. 157

9.4.1 H-matrix pruning. 158

9.4.2 Tests for C2H4. 164

9.4.3 Calculations for CH3NO2. 167

9.5 Summary and Conclusions. 172

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