  Computationally-efficient semilocal approximations of density functional
theory at the level of the local spin density approximation (LSDA) or
generalized gradient approximation (GGA) poorly describe weak interactions. We
show improved descriptions for weak bonds (without loss of accuracy for strong
ones) from a newly-developed semilocal meta-GGA (MGGA), by applying it to
molecules, surfaces, and solids. We argue that this improvement comes from
using the right MGGA dimensionless ingredient to recognize all types of orbital
overlap.
