In tetravalent network materials like quartz or ice the main interactions are the strong bonds that connect each vertex to four others and the repulsive forces that keep nonbonded vertices further apart. Computer simulations, in which those interactions are modeled by square wells, show that they are sufficient to account for the following behavior: The stability of icelike crystals, with tetrahedral bond angles, at low temperature; endothermic melting of the crystal to a denser waterlike amorph at intermediate temperatures; exothermic collapse of the supercooled crystals at high pressure, to a glass, without breaking any bonds; expansion on cooling of both the crystal and the amorph; and a line of spinodal instabilities in the waterlike supercooled amorph where the compressibility, heat capacity, and negative expansivity diverge. The spinodal is associated with correlated low-density, low-energy clusters, characterized by an excess of near tetrahedral bond angles. (C) 1997 American Institute of Physics.