Semiempirical cyclic cluster

As in the FPLAPW method the further increasing of the superceU is sufficiently difficult that the dependence of the results on the superceU size was not studied in [718]. Such a study was made in semiempirical cyclic-cluster calculations of Li and... 

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. 

Hybrid approaches combining ab-initio or DFT and semiempirical approaches have become popular. As an example, we can refer to LEDO (hmited expansion of differential overlap) densities application to the density-functional theory of molecules [262]. This LEDO-DFT method should be well suited to the electronic-structure calculations of large molecules and in the anthors opinion its extension to Bloch states for periodic structures is straightforward. In the next sections we discuss the extension of CNDO and INDO methods to periodic stmctures - models of an infinite crystal and a cyclic cluster. 

The considered CNDO method for periodic systems formally corresponds to the model of an infinite crystal or its main region consisting of L primitive cells. This semiempirical scheme was also apphed for the cychc-cluster model of a crystal allowing the BZ summation to be removed from the two-electron part of matrix elements. In the next section we consider ZDO methods for the model of a cyclic cluster. 

Semiempirical LCAO Methods in Cyclic-cluster Model... 

The cyclic-cluster approach was implemented in the semiempirical SCF-MO method SINDOl [319] and its recent modification MSINDO [301] and has been successfully applied for bulk and surface properties of oxides [302,320]. 

The application of molecular orbital theory to predict structures and properties of C species started with the pioneering work of Pitzer and dementi in 1959 [225]. By using semiempirical MO theory, these authors found the cumulenic linear conformation to be the most stable geometry for except possibly for very large values of n. They predicted that the n-odd clusters would have closed-shell structures and lower energies than the singlet n-even species. Cyclic molecules were found to be unimportant under most conditions. 

---