Critical examination of approximate <scp>LCAO</scp><scp>MO</scp> methods. I. Reasons for the failure of the <scp>CNDO</scp> and <scp>INDO</scp> methods in theoretical conformational analysis of conjugated compounds

Abstract

Abstract It is well known that the CNDO and INDO methods fail in the field of theoretical conformational analysis of conjugated compounds. The NDDO approximation, however, can be applied very successfully to this problem. In this paper, a comparative analysis of these three approximations is performed to clarify the reasons for the failure of the former two methods. It is shown that the defects are inherent in their formalisms, and there is no reasonable possibility to compensate them (e.g., by reparametrization) at the level of the CNDO and INDO approximations. The central inadequacy is the oversimplified treatment of the two‐center exchange interaction. In the case of molecules containing heteroatoms with lone‐pair electrons the known defects concerning the description of Coulombic interactions of anisotropic charge distributions can additionally influence the results. To overcome these shortcomings, at least an extension of the formalism to the level of the NDDO method is necessary.