Theoretical chemistry is the examination of the structural and dynamic properties of molecules and molecular materials using the tools of quantum chemistry, equilibrium and nonequilibrium statistical mechanics and dynamics. Molecular orbital calculations applied to organic and inorganic molecules, solids and surfaces have illuminated profound connections between inorganic and organic chemistry and solid-state physics. <br><br>Statistical mechanical studies of phase transitions, critical phenomena, and interfaces are yielding a fundamental understanding of porous media, microemulsions and polymer solutions. Investigations of energy flow in vibrationally excited molecules contribute to a microscopic understanding of chemical reactivity. Important advances have been made in predicting the structure and dynamics of biomolecules, simulating and interpreting spectroscopic lineshapes, assessing traditional models of chemical kinetics and predicting chemical reactivity by ab initio methods.