The fourth volume of the Collected Works is devoted to Wigner's contribution to physical chemistry, statistical mechanics and solid-state physics. In his annotation to the first part, N. Balazs clearly points out Wigner's role in early quantum chemistry and applied statistical mechanics. One corner stone was his introducing of what is now called the Wigner function, and his paper on adiabatic perturbations foreshadowed later work on Berry phases. Although few in number, Wigner's articles on solid-state physics laid the ground for the modern theory of the electronic structure of metals. W. Kohn writes about this in his annotation to Part 2 of the book and he also points out Wigner's great influence in the development of solid-state theory.

I Physical Chemistry.- Wigner on Physical Chemistry.- On Electrical Phenomena in Sols with Rod-Shaped Particles / Über elektrische Erscheinungen bei Stäbchensolen.- The Lattice Structure of Rhombic Sulfur / Die Gitterstruktur des rhombischen Schwefels.- Formation and Decay of Molecules / Bildung und Zerfall von Molekülen.- On the Interference of Eigenvibrations as a Cause of Energy Fluctuations and Chemical Reactions / Über die Interferenz von Eigenschwingungen als Ursache von Energieschwankungen und chemischer Umsetzungen.- The Statistics of Composite Systems According to the New Quantum Mechanics / Statistik zusammengesetzter Systeme nach der neueren Quanten-Mechanik.- On the Rate Constants of Exchange Reactions / Über die Geschwindigkeitskonstante von Austauschreaktionen.- On Crossing Over Potential Barriers in Chemical Reactions / Über das Überschreiten von Potentialschwellen bei chemischen Reaktionen.- On the Quantum Correction for Thermodynamic Equilibrium.- Contributions to the Theory of the Neutron / Beiträge zur Theorie des Neutrons.- On the Paramagnetic Transition of Para-Ortho-Hydrogen. III / Über die paramagnetische Umwandlung von Para-Orthowasserstoff. III.- On a Modification of the Rayleigh-Schrödinger Perturbation Theory.- Calculation of the Rate of Elementary Reactions of Light and Heavy Hydrogen.- On the Rate of Chemical Reactions.- Calculation of the Rate of Elementary Association Reactions.- The Transition State Method.- Some Quantum-Mechanical Considerations in the Theory of Reactions Involving an Activation Energy.- Some Remarks on the Theory of Reaction Rates.- On the Calculation of the Distribution function.- Electromagnetic Field Expansions in Loss-Free Cavities Excited Through Holes.- The Problem of Multiple Scattering.- Derivations ofOnsager's Reciprocal Relations.- Lower Limit for the Energy Derivative of the Scattering Phase Shift.- Review of Collision Theory.- Quantum-Mechanical Distribution Functions Revisited.- Quantum-Mechanical Distribution Functions: Conditions for Uniqueness.- Some Properties of a Non-negative Quantum-Mechanical Distribution function.- Distribution Functions in Physics: Fundamentals.- Manifestations of Bose and Fermi Statistics on the Quantum Distribution Function for Systems of Spin-0 and Spin-1/2 Particles.- Covariant Phase-Space Representation for Harmonic Oscillators.- The General Properties of the Distribution Function and Remarks on its Weakness.- Covariant Phase-Space Representation and Overlapping Distribution Functions.- II Solid State Physiscs.- Wigner on Solid State Physics.- On the Constitution of Metallic Sodium.- On the Constitution of Metallic Sodium. II.- On the Interaction of Electrons in Metals.- Theory of the Work Functions of Monovalent Metals.- On the Possibility of a Metallic Modification of Hydrogen.- On the Structure of Solid Bodies.- Theory of the Brillouin Zones and Symmetry Properties of Wave Functions in Crystals.- On the Constant A in Richardson's Equation.- Effects of the Electron Interaction on the Energy Levels of Electrons in Metals.- Qualitative Theory of the Cohesion in Metals.- Bibliography of Papers on Solid State Physics.- Papers Reprinted in Volume IV.- Papers Related But Reprinted in Other Volumes of The Collected Works.- Papers Related But Not Reprinted in The Collected Works.