SeqQuest: A density functional electronic structure code [1]
Peter A. Schultz
Computational Materials and Molecular Biology Dept.
Sandia National Laboratories, Albuquerque, NM 87185

I. SeqQuest: A high-performance Gaussian-based density functional code
I describe the implementation of a high-performance density functional code using pseudopotentials and a high-quality Gaussian basis. All the necessary gradients needed for the full GGA functionals are evaluated efficiently and exactly using a hybrid grid/analytic method that does not require a White-Bird approach. The total Hamiltonian build scales as O(N), both in principle, and in actual practice. Hence, geometry- and cell-minimized DFT calculations - both LDA and GGA, spin-polarized or spinless - with hundreds, even thousands, of atoms are easily tractable on single-processor workstations. With very large (and fast) calculations enabled using very modest computational resources, the quality of the DFT becomes the principal limiting factor in the fidelity of calculations in representing physical systems.

II. Correct Coulomb "functionals" for supercell calculations.
Density functional calculations for molecules or defects in extended systems frequently invoke the supercell approximation and solve for an electrostatic potential within periodic boundary conditions. For systems that are charged, or contain dipole (or higher) moments, this prescription introduces errors due to the interactions between the system and the multipole moments of its periodic images. The magnitude of these errors can be larger than the physical energy scale of interest - the band gap energy - and thereby render shortcomings in the DFT moot. I describe a method that treats the local electrostatic potentials correctly by eliminating the spurious interactions across supercell boundaries. A local density is constructed matching the multipole moments of the full density, and its potential is evaluated using isolated boundary conditions within the volume of the supercell. With this model density removed from the full density, the remainder density no longer contains moments with long-range potentials and its electrostatic potential can be evaluated accurately using periodic boundary conditions. The method is simple, effective, and independent of cell shape.

[1] Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy, under Contract No. DEAC0494AL85000.