Abstract <p>The numerical values of the van der Waals interaction constant <i>C</i>₆ are determined between alkaline earth atoms in singlet Rydberg |<i>n</i>¹<i>S</i>₀〉 states with large principal quantum numbers, <i>n</i> &gt; 20. The results of calculations using the semiempirical quantum defect theory (QDT) method are approximated by quadratic polynomials. The polynomial coefficients are tabulated and can be used for simplified quantitative estimates of interatomic interaction-induced energy shifts, determination of the Rydberg state excitation blockade radius, and limitations on the wavelengths of radiation forming Rydberg lattices for group IIA atoms. The numerical values of the contributions of various diatomic states to the constant <i>C</i>₆ are calculated. States providing the main contribution, analogous to that of the Förster resonance, are identified.</p>