Properties, Spectroscopy, and Photochemistry of Molecules and Materials

We have used linear-response coupled-cluster methods, along with other ab initio approaches, to calculate molecular multipole moments and (hyper)polarizabilities and the effect of nuclear motion on these properties. We have also used first-principles theories to obtain rovibrational, electronic, and rovibronic spectra of several molecules and weakly bound species, including van der Waals precursors of photo-induced charge-transfer reactions, and explained the blue shift in the electronic spectrum of acetylacetone due to the formation of the non-chelated species after the UV irradiation of its chelated form. We have demonstrated that the lowest excited state of methylcobalamin should be interpreted as metal-to-ligand charge-transfer excitation and that azulene possesses the doubly excited state below the ionization threshold, which can drive multi-photon ionization experiments related to Rydberg fingerprint spectroscopy. We have also provided definitive information about structural, electronic, and spectroscopic properties of several organic biradicals and small metal nanoparticles, including, for example, beryllium, magnesium, silver, and gold clusters. We have contributed to the design of an improved Reax force field for simulating various properties of the lithium-oxygen material, such as lattice parameters, cohesive energy, bulk modulus, elastic constants, and fracture, and employed our local correlation coupled-cluster methods, combined with QM/MM, to examine the processes of etching and diffusion of atomic oxygen on the silicon surface. Some of our best coupled-cluster methods, combined with QM/MM and spectral simulations, have allowed us and our experimental and theoretical collaborators to understand the highly complex singlet and triplet manifolds involved in the photophysics of the V_B^- defect in hexagonal boron nitride.