Based on density functional theory and electronegativity equalization princ
iple, a an model, which considers the structure of double bond for the firs
t timet is developed, In this model the double bond is partitioned into one
sigma bond region and four pi bond regions. The atom-bond electronegativit
y equalization method plus sigma pi model(ABEEM sigma pi) is then proposed
for the calculation of charge distribution and charge polarization in large
molecules. It has been shown that the results of charge distribution in la
rge molecules obtained by ABEEM sigma pi model agree with those obtained by
ab initio method very well. In addition, the model can be used to explain
the region-selectivity of Diels-Alder reaction successfully. All these indi
cate that ABEEM sigma pi model is reasonable and reliable.