Small rings and amorphous tetrahedral carbon

We apply first-principles density-functional calculations to study strain in dense amorphous tetrahedral carbon (a-tC). While the large strain present in small-ring structures, particularly three-member rings, could argue against their existence in a-tC, we demonstrate, based on energetic arguments, that strained small (three- and four-member) rings are plausible topological microstructural elements. We present two bulk carbon structures made up entirely of fourfold-coordinated atoms: the first with every atom in one three-member ring, the second with every atom in one four-member ring. Calculations show these bulk ring structures are relatively low in energy, only 0.37 and 0.23 eV/atom above diamond, respectively. This computed strain energy is much less than that present in recent models for a-tC. We examine properties of these structures with the intention to provide benchmark calculations for more approximate models, and to investigate the impact small rings might have on the properties of a-tC. We use a recently developed linear-response algorithm to compute phonon spectra for these ring structures. {copyright} {ital 1999} {ital The American Physical Society}