Electronic energies are calculated for a Hubbard model on the ${\mathrm{C}}_{60}$ molecule using projector quantum Monte Carlo (QMC) methods. The calculations are performed to an accuracy high enough to determine the pair-binding energy for two electrons added to neutral ${\mathrm{C}}_{60}$. The method itself is checked against a variety of other quantum Monte Carlo methods as well as the exact diagonalization for smaller molecules. The conclusion is that the ground state with two extra electrons on one ${\mathrm{C}}_{60}$ molecule is a triplet, and, over the range of parameters where QMC is reliable, it has a slightly higher energy than the state with electrons on two separate molecules, so that the pair is unbound.

• Received 16 November 2004

DOI:https://doi.org/10.1103/PhysRevB.71.165436