Theoretical Chemistry: 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. Theoretical chemistry unites principles and concepts common to all branches of chemistry. Within the framework of theoretical chemistry, there is a systematization of chemical laws, principles and rules, their refinement and detailing, the construction of a hierarchy.