The SCF-LCAO-MO Method and Extensions

A rigorous variational calculation on a system involves the following steps  

Select some mathematical functional form lr as the trial wavefunction. This form should have variable parameters. 

The simple and extended Hiickel methods are not rigorous variational calculations. Although they both make use of the secular determinant technique from linear variation theory, no hamiltonian operators are ever written out explicitly and the integrations in Hij are not performed. These are semiempirical methods because they combine the theoretical form with parameters fitted from experimental data. 

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The SCF-Xa Scattered-wave Method.—An alternative to the LCAO-MO method for molecules is the Xa scattered wave or multiple-scattering Xa method, which was suggested less than 10 years ago114 and now has an extensive literative. The basic theory has been refined in several directions115 and there are several excellent reviews.116-180 The method is based on the scattered-wave method in solids181 and aims to solve the Fock equation,... 

In the present paper, first we investigate the photoionization cross sections for atomic orbitals calculated with different scaling parameters of exchange-correlation potential, and for those of different oxidation states, namely different charge densities. We discuss the effect of the variation of the spatial extension of the atomic orbital on the photoionization cross section. Next we make LCAO (linear combination of atomic orbitals) molecular orbital (MO) calculations for some compounds by the SCF DV-Xa method with flexible basis functions including the excited atomic orbitals. We calculate theoretical photoelectron spectrum using the atomic orbital components of MO levels and the photoionization cross sections evaluated for the flexible atomic orbitals used in the SCF MO calculation. The difference between the present result and that calculated with the photoionizaion cross section previously reported is discussed. 

An extensive study has been performed concerning the basicity of aminoazines using the SCF MO LCAO method.165 Moreover, attention has been paid to the influence of various factors influencing the basicity of azines196 and to the relationship between the protonation and solvation entropy.167... 

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