Abstract
Both relativistic and nonrelativistic analytical calculations of the off-diagonal matrix elements between excited |1s3s 3S1⟩ and |1s3d 3D1⟩ states of He-like atoms have been performed, taking into account the spin-spin interaction of the first order of perturbation theory. The considered values of nuclear charge Z range from Z = 2 to Z = 100. The nonrelativistic results for neutral helium agree well with earlier numerical estimates. The relativistic approach of evaluation of the off-diagonal matrix elements of He-like multicharged ions is analyzed in some details, using the hydrogen-like approximation for the basis wave functions. A precise evaluation of the one-photon exchange correction to the |1s3s 3S1⟩ and |1s3d 3D1⟩ energies is made.
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