Semiempirical LCAO methods

The semiempirical LCAO methods for periodic systems are considered in the next chapter. 

Semiempirical LCAO Methods for Molecules and Periodic Systems... 

Semiempirical LCAO Methods in Cyclic-cluster Model... 

A comparative study of gas-phase and liquid-phase CH acidity of a-substituted cyclopropanes has verified the correctness of an electrostatic model for the effect of solvation on equilibrium acidity 127 among variants of the LCAO MO method, only the semiempirical AMI method accurately predicted the proton affinity of the conjugate carbanions effects of solvation on protonation rates have been desegregated. 

Manfred Scholz and Hans Joachim Koehler, Quantum Chemical Method of Approximation and Its Use in Organic Chemistry Introduction to the Semiempirical LCAO MO Method, Huethig, Heidelberg, 1981. 

The 7r-electron structures and energies of the singlet tt-tt transitions for a number of l//-pyrrolo[l,2-a jimidazoles (39), l/f-pyrrolo[l,2-f>]-s-triazoles (40) and l//-pyrrolo[2,l-c]-s-triazoles (41) were calculated by the MO LCAO method within the semiempirical self-consistent field (SCF) approximation. A comparison of the data shows that the maximum... 

Semiempirical quantum-chemical methods can be subdivided into two groups. The first one covers so-called simple MO LCAO methods, of which extended Hiickel treatment (EHT) (7) and its modification by Anderson (ASED) (5), additionally taking into account core repulsion, are most widely used in chemisorption computations. Anderson s improvement of EHT was aimed at obtaining more reliable values of the total energy and at gaining an opportunity to optimize the geometry of chemisorption structures. The method was mainly used in calculations of chemisorption and catalysis on metals. Its validity for oxide systems, with their rather highly ionic bonds, is formally less justified. 

The second group of semiempirical MO LCAO methods is constituted by zero-differential overlap methods (9). Two subgroups can be specified here which somewhat conventionally may be called physical and chemical. The former involves CNDO/2, INDO, and some of their modifications, for example, CNDO/S. These methods are directed to the calculations of the electron characteristics charge distributions, dipole moments, polarizabili-... 

During the past several years1 there has been an intensive emphasis on calculating such diagrams by nonempirical LCAO-MO-SCF techniques as well as by semiempirical LCAO-MO methods. (Most of these results postdate... 

The simulations in this study were done using the semiempirical MO method PM3(Stewart, 1989 Stewart, 1989 Stewart, 1991) within the context of the QM/MM coupled potential recently implemented within our lab.(Hartsough and Merz, 1995) A modified version of MOP AC 5.0 (Besler, Merz et al., 1990 Merz and Besler, 1990), was used for the QM calculations and the driver MD program was a customized version of AMBER (Pearlman, Case etal., 1991). The PM3 Hamiltonian was selected because of its superior correlation with experimental heats of formation for the compounds studied herein (Stewart, 1989). From a methodological perspective, AMI (Dewar, Zoebisch et al., 1985), or MNDO (Dewar and Thiel, 1977), or any other LCAO based method could have been used. 

Simple HUckel molecular orbital (HMO) calculations on the pyrrolo[2,l-c][l,2,4]triazole (28) suggest that electrophilic attack would occur most readily at C-10, and this prediction was borne out by observations that acid-catalyzed deuteration and bromination by NBS in the dark both occur at this position <85JCR(S)363>. Various reactivity indices have been calculated for a number of pyrrolo[ 1,2-b][, 2,4]triazoles (29) and pyrrolo[2,1 -c][ 1,2,4]triazoles (30) using the MO LCAO method within the semiempirical SCF approximation. These indicate that the 5-position is most susceptible to electrophilic attack, followed by the 7-position <74CHE230>. 

In 1962, Sugano showed that the Seitz model (115) could be interpreted as a molecular orbital model (123), an interpretation that clarifies analysis of these systems. In this interpretation, the absorption bands observed in the TI(I) doped alkali halide system come from the electronic transition aigf a g) hu), but the excited states are still calculated assuming an ionic interaction between the metal and the hgand. Since the thallium-chlorine bond is actually largely covalent, Bramanti et al. (118) modified the approach and used a semiempirical molecular orbital (MO) calculation to describe the energy levels of T1(I) doped KCl. Molecular orbitals were constructed by the linear combination of atomic orbitals (LCAO) method from the 6s and 6p metal orbitals and the 3p chlorine orbitals. Initial calculations were conducted with the one-electron approximation the method was then expanded to include Coulomb and spin-orbit interactions. The results of Bramanti et al. were consistent with experimental... 

The above consideration can be interpreted as deduction of the cyclic cluster model of the infinite crystal when the Hartree-Fock LCAO method (or its semiempirical version with nonlocal exchange) is applied. 

The approximate LCAO methods of quantum chemistry can be divided on empirical (semiquantitative) and semiempirical grounds. 

The electronic structure calculations for C and O impurities in TiN were performed by Ivanovsky, Gubanov, Shveikin and Anisimov (1986a), using the semiempirical MO LCAO method. Analysis of the MOs for the cluster [TiN5XTii2] (X = C, O) showed that the introduction of these impurities... 

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