(a) At ambient temperature and pressure and (b) at high temperature and pressure. The unit cell of the calculation containing 64 water atoms, consisting of hydrogen atoms (white) and oxygen atoms (red), is indicated by a blue box. (c) Comparison of the usual first-principles simulation of the radial distribution function between hydrogen atoms and the first-principles simulation, including the effect of quantum fluctuation of the nucleus, by PIMD software with the results of neutron diffraction experiments.