EFFECTS OF BASIS-SET QUALITY ON THE PREDICTION OF STRUCTURES, ENERGIES, AND PROPERTIES OF AMORPHOUS TETRAHEDRAL CARBON

We systematically investigate the effects of basis set quality on the prediction of a representative amorphous tetrahedral (a-tC) carbon str ucture. In fully self-consistent first-principles calculations, variat ions in the quality of the basis set do result in significant variatio ns in predicted structure. Substantial differences between the calcula tions, ranging from a minimum basis to a high-quality double-zeta plus polarization (DZP) basis, for amorphous carbon stem from two sources. We discover that to properly relax candidate a-tC structures requires a high-quality basis set in the calculation. A minimum basis set is i nadequate to negotiate the transition geometries involved in the makin g and breaking of bonds while relaxing a structure. Relaxation of an a -tC structure proposed by Drabold, Fedders, and Stumm [Phys. Rev. B 49 , 16 415 (1994)] using a minimum, and a DZP basis demonstrates this po int. The minimum basis set calculation leaves the bonding topology ess entially unchanged, while relaxation using a DZP basis removes most of the small rings and triples the number of threefold bonded atoms. In addition, we find that to accurately represent the energetics of highl y defected local structures, as found in a-tC, also requires a high-qu ality basis set. We demonstrate this point by using molecular analogs of local structures found in a-tC. Notable is how Little rebonding and energy separates a-tC structures that have qualitatively different de nsities of threefold atoms.