  We describe a unified approach for the determination of ac-Stark correction
and Kramers-Heisenberg dispersion formula. In both cases the contribution from
infinite intermediate summation appearing in the expression for the
corresponding matrix elements are evaluated exactly in the dipole approximation
for the ground state of hydrogen atom using a variation of the Dalgarno-Lewis
method. The analytical expressions obtained can be efficiently used for the
numerical evaluation of matrix element for all values of incident photon energy
and comparison is made with results obtained by different methods.
