Prediction of crystal structures on the basis of the interatomic forces

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The aim of our project has been to construct and develop the "mineralogy of the coming century". We concentrated our effort on the silica minerals, and our results have been evaluated as of "remarkable ones" by scientific community and journalism abroad. Chief achievements are as following: 1. First-principle pair-potential for SiO_2 system was obtained (Tsuneyuki et al., 1988). Putting the potential parameters into a constant pressure-temperature molecular dynamics program developed by Dr. Nose, 1) pressure-induced amorphization in quartz and cristabaflite, 2) the low-high transition of quartz, and 3) viscosity decrease under pressures in silica melt could successfully reproduced and the mechanisms have been revealed in an atomistic scale (Tsuneyuki et al., 1989 and 1990). 2. A silica polymorph denser than stishovite (of the rutile structure) was shown to be stable at very high pressures by the local density funetional calculation using the full- potential linear augmented plane wave approximation (Park et al., 1988). 3. Several new structures of SiO_2, characterized by coexistence of Si atoms in four- and six-fold coordinations has been recognized (and crystallographically described) during the course of high-pressure loading computer experiment on quartz and cristobalite. This suggests the possibility that "novel" structured compounds may be obtained by thermodynamically non-equilibrated runs rather than usual, equilibrated runs(Tsuneyuki et al., 1990; Tsuchida and Yagi, 1989).Due to the limitation in time, development of quantum-mechanical molecular dynamics program (an extension of the methodology suggested by Carr and Parinello) has not been finished; this being the most urgent task which has to be postponed by the present project.