Weak Bonds from a Semilocal Density Functional with the Right Ingredients

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: 1) the binding energy curve of graphene adsorbed on a Ni (111) surface, where both chemi- and physisorption minima are present; 2) stacking of DNA/RNA nucleobases. We argue that this improvement comes from using the right MGGA dimensionless ingredient to recognize all degrees of orbital overlap.