ИСТИНА

The relative stability of the two most important forms of elemental carbon, diamond and graphite, is readdressed from a new perspective. Unlike previous theoretical studies mostly using numerical methods, we propose analytic models, based on deductive quantum molecular mechanics, to get an insight into the fundamental reasons for the quasi-degeneracy of these allotropes with very different bonding patterns. We directly derive the relative energies of the allotropes and prove several general statements. Our analysis provides a qualitative picture predicting a quasi-degenerate electronic ground state for graphite and diamond at 0 K. It also gives numerical estimates of the energy difference which are in a surprisingly good agreement with experimental data and recent results of hybrid DFT modeling, although obtained with a much smaller numerical effort. An attempt to extend this treatment to the lowest energy allotropes of silicon proves to be very successful as well.