Molecular dynamics simulation of isobaric heating of an H2O lattice wi
th diamond cubic structure reveals a distinct two-step sequence of hyd
rogen-bond breaking in a temperature range clearly below the melting p
oint. Partial breakage, signaled by an abrupt increase in local atomic
coordination and the coupling of intermolecular torsions with intramo
lecular angle bending, first occurs and leaves the lattice in a fragme
nted but still ordered state. This is then followed by full breakage a
t a temperature about 20-30 K higher, bringing the lattice to an orien
tationally disordered state characterized by a vibrational spectrum si
milar to that of water.