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Summary: VOLUME 83, NUMBER 13 P H Y S I C A L R E V I E W L E T T E R S 27 SEPTEMBER 1999
The Nature of Attraction between
Like-Charged Rods
The origin of attractive interactions in polyelectrolyte
solutions has provoked a lot of attention. In recent Letters
Ha and Liu [1] attempted to study this interesting problem
using a two-state model originally introduced by Oosawa
and Manning. In the context of this model the counterions
are treated as either free or condensed.
To calculate the free energy of interaction between
two rodlike polyions the authors performed the Hubbard-
Stratonovich transformation to map the Hamiltonian of the
interacting charged rods onto an effective field theory. The
authors proceeded to linearize the resulting action, finish-
ing up with the so-called Gaussian approximation. The
attraction between two charged rods, within this approxi-
mation, arises as a result of correlations in thermal fluctua-
tions. Although nowhere stated explicitly, the Gaussian
approximation, being a form of high-temperature expan-
sion (weak coupling), is only valid for a2
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