Before revision:  Our theory fits experimental datawell. The strength of DNA-DNA short range attraction  , mediated by Mg^{+2}, is found to be -0.004 k_BT per nucleotide base  .
After revision:  By fitting our theory to available experimental data, the strength of DNA-DNA short range attraction , mediated by Mg^{+2}, is found to be -0.004 k_BT per nucleotide base .