  Many important ultrafast phenomena take place in the liquid phase. However,
there is no practical theory to predict how liquids respond to intense light.
Here, we propose an $ab~initio$ accurate method to study the non-perturbative
interaction of intense pulses with a liquid target to investigate its
high-harmonic emission. We consider the case of liquid water, but the method
can be applied to any other liquid or amorphous system. The liquid water
structure is reproduced using Car-Parrinello molecular dynamics simulations in
a periodic supercell. Then, we employ real-time time-dependent density
functional theory to evaluate the light-liquid interaction. We outline the
practical numerical conditions to obtain a converged response. Also, we discuss
the impact of nuclei ultrafast dynamics on the non-linear response of system.
In addition, by considering two different ordered structures of ice, we show
how harmonic emission responds to the loss of long-range order in liquid water.
