In: Handbook of Computational Chemistry Research ISBN: 978-1-60741-047-8 Editors: C.T. Collett and C.D. Robson, pp. 97-125 © 2010 Nova Science Publishers, Inc. Chapter 3 CONSTRAINING OPTIMIZED EXCHANGE Marcel Swart, a,b,c Miquel Solà b and F. Matthias Bickelhaupt a a Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands b Institut de Química Computacional and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Catalunya (Spain) c Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Catalunya (Spain) Abstract Several recent studies (J. Phys. Chem. A 2004, 108, 5479; J. Comput. Chem. 2007, 28, 2431) have shown impressive results when replacing the non-empirical PBE density functional by the empirical OPBE or OLYP functionals, i.e. replacing the PBE exchange functional by Handy and Cohen’s OPTX functional. To investigate the origin of the improvements, we have placed constraints from the non-empirical PBE exchange functional on the empirical OPTX exchange functional, and tested the performance of the resulting constrained functionals for several characteristic chemical properties. The performance of the new functionals is tested for a number of standard benchmark tests, such as the atomization energies of the G2 set, accuracy of geometries for small molecules, atomic exchange energies, and proton affinities of anionic and neutral molecules. Furthermore, the new functionals are tested against a benchmark set of nucleophilic substitution S N 2 reactions, for which we have recently compared DFT with high-level coupled cluster CCSD(T) data (J. Comput. Chem. 2007, 28, 1551). Our study makes clear that the performance depends critically on the number of constraints, and on the reference data to which the constrained functionals are optimized. For each of these properties studied, there is at least one functional that performs very well. Although a new promising functional (ML ff OLYP) emerged from the set of constrained functionals that approaches coupled-cluster accuracy for geometries and performs very well for the energy profile of S N 2 reactions, there is no one of the newly constructed functionals that performs equally well for all properties. Keywords: Density Functional Theory – Exchange functional – Geometry – Reactivity – Properties.