  We study optimal double helices with straight axes (or the fattest tubes
around them) computationally using three kinds of functionals; ideal ones using
ropelength, best volume packing ones, and energy minimizers using two
one-parameter families of interaction energies between two strands of types
$r^{-\alpha}$ and $\frac1r\exp(-kr)$. We compare the numerical results with
experimental data of DNA.
