Weinhold’s earlier (1969-80) research interests centered on upper and lower bounds for quantum-mechanical properties, the metric geometry of equilibrium thermodynamics, and complex-coordinate rotation theory of autoionizing resonances.
More recent research has primarily focused on the development of the Natural Bond Orbital (NBO) method and its application to torsional barriers, hydrogen bonding, hypervalency, and other stereoelectronic and bonding phenomena. Related studies also led to development of the Quantum Cluster Equilibrium (QCE) theory for liquid water and other supramolecular species. His influence is indicated by widespread adoption of NBO methodology in modern electronic structure programs (currently including Gaussian, Jaguar, GAMESS, ADF, Terachem, NWChem, PQS, Q-Chem, Columbus, and Spartan) and numerous applications in the chemical literature (currently, 1,100+ per year).
His recent research monographs include Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective (Cambridge University Press, 2005; with C. R. Landis), Classical and Geometrical Theory of Chemical and Phase Thermodynamics (Wiley, 2009), and Discovering Chemistry With Natural Bond Orbitals (Wiley, 2012; with C. R. Landis).