We calculated the electrostatic potential (ESP) and electric field (EF) of periodic liquid water systems using the quantum chemistry software package, Crystal. We proposea method to obtain atomic partial charges rapidly for periodic systems based on first-principles calculations. In this method, the average electrostatic potential *φ*^{mean}, which is introduced to meet the periodic boundary condition, is taken as a parameter during the least squares fitting of the ESP from first-principles calculations and used in the Ewald summation. A comparison of the two methods, i.e., ESP and EF fitting, reveals that the relative root mean-square deviation (RMS) of the former method is only 2%-3%, which is one order of magnitude smaller than that of the latter method. In addition, the distribution of the derived atomic partial charges and dipole moments for the water system are discussed using four charge restrained fits.