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506. AMPAC: Austin Method 1 Package (Version 2.1)
by Dewar Research Group, Chemistry Department, University of Texas, Austin, Texas 78712 AMPAC represents the current culmination of a project that has occupied the Dewar group for twenty years and which still continues, i.e., the development of quantum mechanical procedures accurate enough, and simple enough, to serve as a practical tool for chemists to use in their own research. For a brief history, see Ref. 1. Many contributed to the development of the underlying theoretical procedures and their application to a wide variety of chemical properties and to the development of corresponding computer programs. A few years ago, one of us (J.J.P.S.), as a member of the Dewar Research Group, undertook the task of rewriting and reorganizing these programs into a single package, a project which also led to a major increase in their efficiency, addition of further options, and greatly simplified input. This package (MOPAC)2 has gained wide acceptance. Recently, we have developed3 a "third-generation" treatment which seems to represent a major advance over MINDO/3 or MNDO. Because of the continued confusion of our procedures with other much inferior semiempirical methods, we decided on a change from the previous "NDO" nomenclature. The new treatment was therefore termed AM1 (Austin Model 1). The name of the present package has been changed correspondingly to AMPAC. It represents an enhanced version of MOPAC, including AM1. MINDO/3 and MNDO are retained, for completeness, because there are areas where each is particularly effective and because MNDO parameters are currently available for a much wider range of elements than for AM1. A full configuration interaction treatment had been developed earlier.4 This was inadvertently omitted from MOPAC, where only C.I. options were available. Full C.I. is now included in AMPAC. The MNDO Hamiltonian has been extended to include d orbitals, and preliminary parameters are included for chromium. It should be stressed that these are preliminary; our hope is that their use by others may help to show up their deficiencies. We know already that they perform poorly for p complexes. MNDO parameters are now included for H, Be, B, C, N, O, F, Hg, Al, Si, Ge, Sn, Pb, P, S, Cl, Br, and I. AM1 parameters are currently available for C, H, O, N, F, Cl, Br, and I. While MINDO/3 and MNDO had previously been extended to linear polymers,5 no option for such calculations was included in MOPAC. An improved treatment, applicable to all three Hamiltonians, is included in AMPAC (it represents a subset of MOSOL6). A number of other minor additions and modifications are documented in the AMPAC manual (Appendix D). References:
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