  An accurate ab initio theory of the H-bond structure of liquid water requires
a high-level exchange correlation approximation from density functional theory.
Based on the liquid structures modeled by ab initio molecular dynamics by using
maximally localized Wannier functions as a basis, we study the infrared
spectrum of water within the canonical ensemble. In particular, we employ both
the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient
approximation (GGA) and the state-of-the-art meta-GGA level approximation
provided by the strongly constrained and appropriately normed (SCAN)
functional. We demonstrate that the SCAN functional improves not only the water
structure but also the theoretical infrared spectrum of water. Our analyses
show that the improvement in the stretching and bending bands can be mainly
attributed to better descriptions of directional H bonding and the covalency at
the inter- and intramolecular levels, respectively. On the other hand, better
agreements in libration and hindered translation bands are due to the improved
dynamics of the H-bond network enabled by a less structured liquid in the
experimental direction. The spectrum predicted by SCAN shows much better
agreement with experimental data than the conventionally widely adopted PBE
functional at the GGA level.
