Gaussian 03 Online Manual
Use sparse matrix storage for performance enhancement of large calculations (above around 400 atoms) . The keyword's option allows you to specify the cutoff value for considering matrix elements to be zero.
Energies and gradients for AM1, Hartree-Fock and DFT methods (closed shell calculations). It is useful for AM1 calculations of more than 200 atoms. This keyword may also be used within method specifications for ONIOM layers.