Construction of an optimal GGA functional for molecules and solids

The specialized exchange-correlation functionals of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] (WC) and Hammer et al. [Phys. Rev. B 59, 7413 (1999)] [revised Perdew, Burke, and Ernzerhof (RPBE)] yield good results for either the lattice constants of solids or the atomization energies of molecules, respectively, but are rather poor for the opposite quantity. By combining the WC and RPBE functionals, we construct a functional that performs equally well for both molecules and solids. Our proposed functional, which is still in the form of the simple generalized gradient approximation, can thus be applied efficiently to systems that involve both finite and infinite systems, a case that is crucial, for example, in heterogeneous catalysis studies. Therefore, the chemisorption of CO on transition-metal surfaces was considered, and it is shown that our functional gives improved results.