Structural and Vibrational Properties of Liquid Water from van der Waals Density Functionals.

We present results for the structural and vibrational properties of the water molecule, water dimer, and liquid water at the experimental equilibrium density, as obtained with several van der Waals density functionals. The functional form originally proposed by Dion et al. [ Phys. Rev. Lett. 2004 , 92 , 246401 ], with an appropriately chosen local exchange functional, yields a description of the liquid superior to that of the semilocal functional PBE. In particular, a specific choice of the local exchange functional (optB88) fitted to quantum chemistry calculations yields the best agreement with experimental results for pair correlation functions although it is slightly inferior to other van der Waals functionals in describing infrared spectra. When using optB88, liquid water displays a hydrogen-bonded network less tightly bound than when using the PBE approximation. The performance of optB88 is definitely inferior to that of the PBE0 hybrid functional for the isolated molecule but only moderately so for the liquid. However, the computational cost of optB88 is much less than that of hybrid functionals; therefore the use of optB88 appears to be a sensible alternative to calculations implying the evaluation of the Fock operator, in cases when simulations of large systems are required.