First-principles methods are essential for calculating the atomic polarization with high accuracy. This chapter introduces the basics of density functional theory (DFT). First, we review the Born-Oppenheimer (BO) approximation, a fundamental approximation for describing electron lattice systems. The BO approximation decouples the electronic and atomic degrees of freedom, and enables the BO molecular dynamics, which optimizes electronic structure at each atomic configuration. Then, we outline the Hohenberg-Kohn theorem and Kohn-Sham scheme, the foundation of the DFT for electronic structure calculations.