Ideal, best packing, and energy minimizing double helices

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 $\frac{1}{r}\exp (-kr)$. We compare the numerical results with experimental data of DNA.