a Study of Buckminsterfullerene Using New Pseudopotentials.

We present pseudopotential plane-wave local-density calculations of the electronic and structural properties of solid C_{60} (fullerite). The calculated molecular bond lengths, lattice constant, bulk modulus, enthalpy of formation, and the equation of state for compression are in good agreement with experiment. The shape of the theoretical density of states is in excellent agreement with the experimental photoemission and inverse photoemission spectra. We also present the calculated band-structure for the states near the fundamental gap. We have made a thorough analysis of the electronic states of C_{60} and found that they can be conveniently classified according to their angular character, and use it to identify the origin of the peaks in the electronic density of states. In order to perform accurate plane-wave converged C_{60} calculations, we have developed a simple procedure to generate first-principles norm-conserving pseudopotentials, which are designed to be smooth and therefore save computational resources when used with a plane-wave basis. We found that these pseudopotentials to be extremely efficient for cases where the plane-wave expansion has a slow convergence, in particular for systems containing first row elements such as carbon, transition metals and rare-earth elements. The wide applicability of the pseudopotentials are exemplified with plane-wave calculations for copper, zincblende, diamond, alpha-quartz, rutile and cerium. The large number of atoms in the C_ {60} system, imposes a heavy computational load on both storage and cpu time requirements. Therefore, we have investigated the computational requirements for the pseudopotential plane-wave method as a function of the number of atoms per unit cell. For systems containing a large number of atoms the computational load can be reduced if the pseudopotential operator is of a suitable form, such that it can be efficiently calculated in the position representation. The pseudopotentials we have examined include local pseudopotentials, position dependent electron mass pseudopotentials, and separable non-local pseudopotentials.