A linear relation to calculate one particle expectation value r-1 for 1S2 configuration for 2 Z 100

One particle expectation value r-1 for 1S2 configuration for Z =2 to 10 are evaluated by usingHartree-Fock-Roothaan (HFR) wavefunction. A linear relation are determined to describe these results, and this relation have tested to check its ability to calculate one particle expectation value r-1 for atoms with higher atomic numbers (Z = 2 to 100). Results are in good agreement with calculations given by [S.L. Saito Atomic Data and Nuclear Data Tables95, (2009) ,836-870] for Z 20.