Potential Energy Surfaces for Quantum Dynamics Simulations: From ab initio Computations to Vibrational State Determinations

• Author / Creator

  Pradhan,Ekadashi

• Full dimensional potential energy surfaces (PESs) have been constructed using the neural network exponential fitting approach (NN-expnn). High level ab initio energies have been fit to a sum-of-products form (SOP) and the quality of the PESs have been verified by computing vibrational frequencies using the Multi-Configuration Time Dependent Hartree (MCTDH) method. Ground and excited states of CS2, HFCO and HONO have been explored using this NN-expnn technique. The ground state PES and dipole moment surfaces (DMS) for CS2 have been determined at the CASPT2/C:cc-pVTZ, S:aug-cc-pV(T+d)Z level of theory and fit to a SOP form using the NN-expnn method. A generic interface between the NN-expnn PES fitting and the Heidelberg MCTDH software package is demonstrated. The PES has also been fit using the potfit procedure in MCTDH. For fits to the low-energy regions of the potential, the neural network method requires fewer parameters than potfit to achieve high accuracy - global fits are comparable between the two methods. Using these PESs, the vibrational energies have been computed for the four most abundant CS2 isotopomers, compared to previous experimental and theoretical data, and shown to accurately reproduce the low-lying vibrational energies within a few wavenumbers. A local 6D PES for the HFCO molecule was fit in a SOP form using neural network exponential fitting function and validated in MCTDH calculation. The ab initio data were computed at the CCSD(T)-F12/cc-pVTZ-F12 level of theory. The fit PES has a RMSE of 10 cm-1 as compared to the ab initio data up to 10000 cm-1 above the zero point energy. The computed vibrational modes, which cover most of the experimentally measured infrared data, are more accurate that those from the previous MP2-based PES. With this PES, intermolecular vibrational redistribution (IVR) in HFCO and DFCO, the effect of IVR on unimolecular dissociation, and control of IVR using optimal control theory can be studied. A CCSD(T)-F12/cc-pVTZ-F12 computed 6D PES for HONO in the cis-trans region has been fit with the neural network exponential fitting function. The final PES is in SOP form and can directly be used in MCTDH to study spectroscopy and dynamics. The PES is compared with alternate PESs based on CCSD(T)/cc-pVTZ, cc-pVQZ, cc-pV5Z and complete basis set (CBS) extrapolated ab initio data. The vibrational states determined up to 4000 cm-1 for cis- and trans-HONO exhibit very good accuracy when compared to experiment (RMSE of 7.5 cm-1 for cis-HONO and 8.5 cm-1 for trans-HONO). The general NN-expnn fitting method can be applied to other similar 6D molecular systems and has great potential for application to larger systems (9D, etc.) in the future. A global 6D PES was constructed for HFCO using CCSD(T)-F12/cc-pVTZ-F12 ab initio energies. The SOP form of the final analytical surface was used to compute vibrational frequencies using MCTDH. The equilibrium to HF + CO dissociation part of the potential was very accurate, about 10 cm-1 RMSE, compared to recent experiment and theory. The cis-trans-HOCF and HFCO to trans-HOCF regions were also accurate with RMSE of 20 cm-1 compared to the ab initio data. A 6D PES for the HFCO S1 electronic state was determined based on EOM-CCSD/ aug-cc-pVTZ energies. The fundamental vibrational frequencies as computed using MCTDH were in very good agreement with the experimental results. RMSE of 45 cm-1 of the fundamental modes was obtained. The vertical excitation energies were also computed at CASSCF, CASPT2, CASPT2-F12, MRCI and MRCI-F12 levels of theory with different active space, (CAS(8,7), CAS(12,9), and full CAS(18,13)). With this newly constructed PES along with the previous S0 surface (both in SOP form), it is possible to study theoretically stimulated emission pumping (SEP) spectra for the HFCO molecule using MCTDH. Overall, a MATLAB interface (for constructing PESs by directly fitting of ab initio data into SOP form) to the MCTDH software package has been successfully implemented and tested on a diversity of problems. In the future, the present PES fitting method may serve as an alternative to the conventional potfit approach for adopting PESs for use in MCTDH.

• Subjects / Keywords

  • HFCO
  • CS2
  • HONO
  • NN-expnn

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R39Z90Q5J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.