Orbital-free functional with sub-milliHartree errors for slabs

Using principles of asymptotic analysis, we derive the exact leading correction to the Thomas-Fermi kinetic energy approximation for Kohn-Sham electrons for slabs. This asymptotic expansion approximation includes crucial quantum oscillations missed by standard semilocal density functionals. Because these account for the derivative discontinuity, chemical accuracy is achieved at fourth-order. The implications for both orbital-free electronic structure and exchange-correlation approximations are discussed.