Use sparse matrix storage for performance enhancement of large calculations (above around 400 atoms) [34]. The keyword's option allows you to specify the cutoff value for considering matrix elements to be zero.
Loose
Sets the cutoff to 5 * 10-5.
Medium
Sets the cutoff to 5 * 10-7. This is the default for semi-empirical methods.
Tight
Sets the cutoff to 1 * 10-10. This is the default for DFT methods.
N
Sets the cutoff to 1 * 10-N.
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.
