Frequency shift of hyperfine transitions due to blackbody radiation
  We have performed calculations of the size of the frequency shift induced by a static electric field on the clock transition frequencies of the hyperfine splitting in Yb+, Rb, Cs, Ba+, and Hg+. The calculations are used to find the frequency shifts due to blackbody radiation which are needed for accurate frequency measurements and improvements of the limits on variation of the fine structure constant, alpha. Our result for Cs (delta nu E^2=-2.26 times 10^{-10}Hz/(V/m)^2) is in good agreement with early measurements and ab initio calculations. We present arguments against recent claims that the actual value might be smaller. The difference (approx 10%) is due to the contribution of the continuum spectrum in the sum over intermediate states. 
E. J. Angstmann, V. A. Dzuba and V. V. Flambaum