SAM1 (Semiempirical ab initio Model 1)

Tetrahedron, 49 (1993) 5003-5038
Tetrahedron, 50 (1994) 627-638

SAM1 stands for "semiempirical ab initio method 1" and was introduced by MJS Dewar et al in 1993. It is a general purpose semiempirical method. SAM1 represents a new approach to semiempirical method implementations by replacing a key aspect of the AM1 / PM3 / MNDO theoretical model with a different formalism. This key component is the manner is which the the two-electron repulsion integrals (TERIs), a basic term in Hartree-Fock theory, are computed. Previously, this quantity was calculated using the multipole expansion, but SAM1 uses a minimal Gaussian basis set to evaluate these terms directly. Note that the NDDO approximation is still in effect, so only a limited number of the possible integrals are computed. The integral values are scaled as in previous NDDO using parameterized constants and functions. SAM1's performance is superior to AM1, PM3 and MNDO, especially for those heavy main group elements in which d-orbitals are important to properly describing chemical behavior. Parameters are also available for several transition metals.

*Property*	*No.*	*SAM1*	*AM1*	*PM3*
DH_f (kcal/mol)	406	3.97	6.40	5.32
Dipole Moments (m)	196	0.32	0.35	0.40