Electronic absorption spectra from first principles

Methods for simulating electronic absorption spectra of molecules from first principles (i.e., without any experimental input, using quantum mechanics) are developed and compared. The electronic excitation and photoelectron spectra of ethylene are simulated, using the EOM-CCSD method for the electronic structure calculations.; The different approaches for simulating spectra are broadly of two types---Frank-Condon (FC) approaches and vibronic coupling approaches. For treating the vibrational motion, the former use the Born-Oppenheimer or single surface approximation while the latter do not. Moreover, in our FC approaches the vibrational Hamiltonian is additively separable along normal mode coordinates, while in vibronic approaches a model Hamiltonian (obtained from ab initio electronic structure theory) provides an intricate coupling between both normal modes and electronic states.; A method called vertical FC is proposed, where in accord with the short-time picture of molecular spectroscopy, the approximate excited-state potential energy surface that is used to calculate the electronic spectrum is taken to reproduce the ab initio potential at the ground-state equilibrium geometry. The potential energy surface along normal modes may be treated either in the harmonic approximation or using the full one-dimensional potential. Systems with highly anharmonic potential surfaces can be treated and expensive geometry optimizations are not required, unlike the traditional FC approach. The ultraviolet spectrum of ethylene between 6.2 and 8.7 eV is simulated using vertical FC.; While FC approaches for simulation are computationally very efficient, they are not accurate when the underlying approximations are unreasonable. Then, vibronic coupling model Hamiltonians are necessary. Since these Hamiltonians have an analytic form, they are used to map the potential energy surfaces and understand their topology. Spectra are obtained by numerical diagonalization of the Hamiltonians.; The photoelectron spectrum corresponding to the four lowest ionized states of ethylene are simulated using various methods. For the ground ionized state, Duschinsky rotation among the symmetric normal modes is found to play an important role. The FC and vibronic models are refined to capture this effect by locating the center of the Taylor expansion of the potential surfaces used in the models to be the symmetry constrained optimized geometry of the ionized state.