  We extend the recently proposed thermally-assisted-occupation density
functional theory (TAO-DFT) [J.-D. Chai, J. Chem. Phys. 136, 154104 (2012)] to
generalized-gradient approximation (GGA) exchange-correlation density
functionals. Relative to our previous TAO-LDA (i.e., the local density
approximation to TAO-DFT), the resulting TAO-GGAs are significantly superior
for a wide range of applications, such as thermochemistry, kinetics, and
reaction energies. For noncovalent interactions, TAO-GGAs with empirical
dispersion corrections are shown to yield excellent performance. Due to their
computational efficiency for systems with strong static correlation effects,
TAO-LDA and TAO-GGAs are applied to study the electronic properties (e.g., the
singlet-triplet energy gaps, vertical ionization potentials, vertical electron
affinities, fundamental gaps, and symmetrized von Neumann entropy) of acenes
with different number of linearly fused benzene rings (up to 100), which is
very challenging for conventional electronic structure methods. The ground
states of acenes are shown to be singlets for all the chain lengths studied
here. With the increase of acene length, the singlet-triplet energy gaps,
vertical ionization potentials, and fundamental gaps decrease monotonically,
while the vertical electron affinities and symmetrized von Neumann entropy
(i.e., a measure of polyradical character) increase monotonically.
