  We present a general formulation of multichannel quantum-defect theory (MQDT)
for anisotropic long-range potentials. The theory unifies the treatment of
atomic and molecular interactions of all types, and greatly expands the set of
interactions that can be treated and understood systematically, including
complex interactions involving molecules. In one exemplary manifestation, the
theory provides a methodology to make the classification of atomic interactions
based on the Periodic Table quantitative, instead of qualitative, and to
generalize the Table to include molecular classes. Through the concept of
effective potential, the theory further establishes a foundation for new
classes of quantum theories for chemistry and for a broad range of quantum
systems made of either a few or many atoms and/or molecules.
