  In the supercooled regime at elevated pressure two forms of liquid water,
high-density (HDL) and low-density (LDL), have been proposed to be separated by
a coexistence line ending at a critical point, but a connection to ambient
conditions has been lacking. Here we perform large-scale molecular dynamics
simulations and demonstrate that the underlying potential energy surface gives
a strictly bimodal characterization of the molecules at all temperatures as
spatially inhomogeneous either LDL- or HDL-like with a 3:1 predominance for HDL
at ambient conditions. The Widom line, indicating maximum fluctuations,
coincides with a 1:1 distribution. Our results indicate a unified description
of liquid water covering supercooled to ambient conditions in agreement with
recent x-ray spectroscopy and scattering data.
