3 Density Functional Approximations This approach, which allows us to use the resulting exchangecorrelation functionals in exotic systems with some confidence (assuming universality of the functional), may, however, involve constraints that can be questioned, as being not necessarily appropriate for molecular systems -for example, the uniform-gas limit. This constraint was removed by Handy et al. in the OPTX exchange functional and is a reminiscent of the X-alpha functional. This approach, coupled to a correlation functional (e.g., in the OPBE functional), may lead to a good description of spin states. The second category, promoted by the Minnesota team, has led to functionals involving up to 64 parameters. These functionals may be very accurate for -but limited to -small classes of molecular systems and properties. In a paper involving 200 combinations of exchange and correlation functionals 233 ,Mardirossian and Head-Gordon compared the performance of these exchangecorrelation functionals applied to 82 data sets, with and without dispersion corrections, and documented the scattering of the performance among the properties for given classes of molecular systems.