sigma pi scheme in atom-bond electronegativity equalization method

Although, as we know, the traditional ab initio quantum chemical methods can deal with molecules of medium size, it is still a tremendous task to treat very big organic and biological systems. So even today it is still valuable to develop some meaningful relatively simple methods', such as EEM or ABEEM, to see how well and to what extent those kinds of methods can be employed with a huge savings of time to study very large systems. This kind of effort, including what has been done in this paper, may continue for quite a long period, especially considering dynamic problems in the near future. We will develop an atom-bond electro-negativity equalization method plus an (ABEEM an) scheme on the basis of DFT, in which the structure of double bond is explicitly treated in the electronegativity equalization method. In this scheme, the effective electronegativity of a region can be expressed as one valence-state electronegativity (chi*) term, one valence-state hardness (eta*) term, and some other contributing terms coming from other regions' charges in the molecule. By dealing with more than 200 molecules, the parameters chi*, eta*, C, and D in the expression of electronegativity are determined through a regression and least-squares optimization procedure, and their meaning is also discussed. As testing examples, the ABEEM an scheme is used to calculate the charge distributions of C18N3O3H27, C15NO2H21, and thyrotropin-releasing hormone (TRH), and the results show good agreement with those obtained by the ab initio method. However, this scheme is far more time saving and computational-source saving compared to the conventional ab initio method. All of this indicates that the ABEEM an scheme is a reasonable and potential tool for predicting the properties of very large organic and biological systems such as poly-peptides and proteins, etc.