Abstract: We apply the approximation theory to calculate the squared form factors of atoms and molecules in ground state. Then as an example the squared form factors of atomic hydrogen is calculated and compared with the accurate one determined by analytical solution. Then the different approximation methods such as Hatree-Fock and density functional theory as well as different basis sets are tested. The present method can be extended to calculate the squared form factors in the ground states of other atoms and molecules whose wave functions cannot be prepared analytically.

Key words: atomic and molecular physics, squared form factor, Gaussian （ software for calculation）