Based on the principle of electronegativity equalization and density functional theory, a lone pair electron model was developed through partitioning the molecular electron density into atomic electron densities, chemical bond electron densities, and lone pair electrons' densities. The expressions of total molecular energy and the "effective electronegativity'' of an atom or a chemical bond or a lone pair electron are obtained. Then the lone pair electron electronegativity xlp is explicitly defined and the corresponding parameters are calibrated through regression and least-squares optimization procedure. The atom-bond electronegativity equalization method plus lone pair electron model (ABEEM+lp) is then proposed for the direct calculation of the charge distribution and charge polarization in large molecules. In this paper we give the results of the charge distributions in some large molecules obtained by ABEEM+lp. Through comparing the ABEEM+lp charge distributions and ab initio ones, it can be concluded that the lone pair electron model and the parameters are reasonable. In the following paper we will discuss the results of charge polarization in molecules obtained by ABEEM+lp. Through comparing ABEEM+lp charge polarization and ab initio ones, it will be seen that considering lone pair electron explicitly is not only reasonable but also necessary.