For all but the lightest molecules or the very lowest temperatures we have B ≪ ≪ k B T . This suggests we can approximate the sum by replacing the sum over J by an integral of J treated as a continuous variable. ζ ζ rot ≈ ≈ ∫ ∫ 0 ∞ ∞ ( 2 J + 1 ) e − − J ( J + 1 ) B / k B T d J = k B T B .