Aplications of accurate quantum chemical ab initio calculations to solving various chemical problems and include three major areas: (a) Studies of potential energy surfaces for chemical reactions related to combustion, atmospheric and interstellar chemistry combined with statistical calculations of reaction rate coefficients and product branching ratios. (b) Prediction and assignment of absorption and emission spectra based on ab initio calculations of potential energy surfaces for excited electronic states. (c) Quantum chemical modeling of photoabsorption and photoluminescence properties of nanoscale materials.