Improving self-consistent field convergence by varying occupation numbers

One problem commonly encountered in quantum chemistry calculations is the convergence of the self-consistent field (SCF) iteration process. Numerous cases are known in which calculations (both Hartree-Fock and density functional theory), even when using extrapolation techniques, converge extremely slowly or do not converge at all. Many of these cases include molecules that contain transition metals. In this paper, we study two techniques that fractionally occupy orbitals around the Fermi energy during the SCF cycles. These methods use fractionally occupied orbitals to aid in the iterative process, but the occupations at convergence are forced to be ones and zeros. We show how using these fractionally occupied orbitals improves convergence for a number of difficult cases and that there is no significant overhead in the number of SCF cycles for molecules that easily converge with standard techniques. (C) 1999 American Institute of Physics. [S0021-9606(99)30402-5].