We use a quantum chemical approach to examine the phase transition from a π-conjugated stacking system to a covalent bonding system with the goal of elucidating the mechanism of the transition from graphite to diamond under pressure. We adopted the simple model system of three stacking phenalene molecules for the graphite layers and interpreted the transition mechanism toward the diamond structure using the molecular orbital (MO) theory. To treat the phase transition, we introduce the structural self-consistent field (S-SCF) method, by which the molecular structure is efficiently provided under various high pressures by optimizing the intra-molecular parameters of the central molecule under circumferential molecules at various sizes of unit cell. This treatment is applied to examine the mechanisms in the two experimental results reported on the phase transition from hexagonal graphite to diamond and interpreted from the viewpoint of the changes of molecular orbitals through the transformation.
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