We perform ab initio molecular orbital (MO) calculations of the ground and excited state potential energy surfaces (PES) for polyatomic molecules and radicals with applications to the vibrational and electronic spectroscopy, chemical reactions, and photodissociation dynamics related to combustion, atmospheric and interstellar chemistry, etc. Such calculations are possible within chemical accuracy (0.1 eV) both for the ground and excited electronic states using advanced quantum chemical techniques such as the coupled cluster method, CCSD(T), and the multireference configuration interaction method, MRCI, with large and flexible basis sets. The calculations provide a reliable information concerning geometric parameters, energetics, and vibrational frequencies of local minima and transition states on PES for various electronic states and represent an invaluable tool for interpretation of experimental data.
　　From the ground and excited state PES we predict and assign the absorption and emission spectra. The calculations give the vertical excitation energies and oscillator strengths for various electronic transitions as well as the adiabatic excitation energies corresponding to the origins of absorption bands. The intensity of each vibronic (electronic + vibrational) transition is calculated from the electronic transition dipole moment and the Franck-Condon factors. Via ab initio calculations of vibronic coupling between different electronic states we also deal with symmetry-forbidden electronic spectra. Vibronic coupling is relevant to the rate of radiationless internal conversion (IC) of one electronic state to another within the same multiplicity. We carry out ab initio calculations for the IC rates which are critical for understanding the photodissociation mechanisms and dynamics. Once a molecule or a radical is on the ground state PES, in the case of ergodic behavior their dissociation dynamics can be described by the RRKM theory. Based upon the calculated PES for various channels of unimolecular dissociation or bimolecular chemical reactions we use this theory to compute reaction rate constants for different reaction channels. Then, solving the kinetic equations we compute various product yields and branching ratios.