SAM1

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SAM1, or "Semiempirical ab initio Model 1", is a semiempirical quantum chemistry method for computing molecular properties. It is an implementation the general Neglect of Differential Diatomic Overlap (NDDO) integral approximation, and is efficient and accurate. Related methods are AM1, PM3 and the older MNDO.

SAM1 was developed by M.J.S. Dewar and co-workers at the University of Texas and the University of Florida. Papers describing the implementation of the method and its results were published in 1993^[1] and 1994.^[2] The method is implemented in the AMPAC program produced by Semichem

SAM1 builds on the success of the Dewar-style semiempirical models by adding two new aspects to the AM1/PM3 formalism:

Two-electron repulsion integrals (TERIs) are computed from a minimal basis set of contracted Gaussian functions, as opposed to the previously used multipole expansion. Note that the NDDO approximation is still in effect, and that only a few of the possible TERIs are explicitly computed. The values of the explicit TERIs are scaled using empirically-derived functions to obtain experimentally relevant results.
One-center two-electron repulsion integrals (OCTEs) are derived initially to reproduce atomic properties. These values are then fixed and carried forward as further elemental parameterization proceeds.

The performance of SAM1 for C, H, O, N, F, Cl, Br, and I is superior to other semiempirical methods, as shown below. Especially noteworthy are the smaller systematic errors for heats for formation, indicating that SAM1 is more precise for this set of molecules.

SAM1 Performance Compared to AM1 and PM3
Property	N	SAM1	AM1	PM3
ΔH_f (kcal/mol)	406	3.97 (‡3.53)	6.40 (‡6.07)	5.32 (‡4.77)
μ^[3] (Debye)	196	0.32	0.35	0.40
Ionization Potential (eV)	249	0.33	0.62	0.40

One of the main advantages of abandoning the multipole expansion in favor of SAM1's methodology is that d-orbitals may now be more easily treated. This allows calculations using atoms that use these orbitals as a part of their chemistry, such as the heavier main group elements and the transition metals. SAM1 has been parameterized for the elements listed above as well as Si, P, S, Fe, and Cu. Note that Cl, Br, and I did NOT include d-orbitals as part of the parameterization.

[edit] See also

[edit] References

^ M.J.S. Dewar, C. Jie, G. Yu Tetrahedron, 23 (1993) 5003-5038
^ A.J. Holder, R.D. Dennington III, C. Jie, G. Yu Tetrahedron, 50 (1994) 627-638
^ Dipole moment