Exploring the limits of gradient corrections in density functional theory

We have recently proposed a simple and systematic approach to the generation of exchange-correlation functionals in density-functional theory by linear least-squares fitting to accurate thermochemical reference data. In a series of four publications, new functionals with gradient corrections of first and also second order have been found in this way. In the present article we review and summarize our approach, highlighting the common threads and the most extensive fits from the heretofore published studies. © 1999 John Wiley & Sons, Inc. J Comput Chem 20: 63–69, 1999