(color online) Spherical representation (a) and ellipsoidal representation (b) for an RNA or DNA. Here the 58-nt rRNA fragment (PDB code: 1HC8) is shown as a paradigm for spherical and ellipsoidal representations. The spherical and ellipsoidal representations for other nucleic acids used in the work are shown in Fig. S10 in the supplementary material. (c)–(d) Critical charges obtained from PB theory and MC simulations against those from Eq. (4) in divalent (c) or trivalent (d) salt solution. The hollow and solid symbols stand for the data obtained from Eq. (4) with the spherical representation and ellipsoidal representations, respectively. The blue, green and red symbols represent the salt concentrations of 0.1 mM (green), 1 mM (blue), and 10 mM (red) for divalent salt solutions and 0.01 mM (green), 0.1 mM (blue), and 1 mM (red) for trivalent salt solutions, respectively.