  It is well known that equilibrium in a thermodynamic system results from a
competition or balance between lowering the energy and increasing the entropy,
or at least the product of the temperature and entropy. This is remarkably
similar to the Taoist concept of yin, a downward influence, and yang, an upward
influence, where harmony is established by balancing yin and yang. Entropy is
due to structure, which is largely determined by core repulsions or particle
size whereas energy is largely determined by longer range attractive
interactions. Here, this balance between energy and entropy or particle size is
traced through the theory of simple fluids, beginning with Andrews and van der
Waals, the subsequent developments of perturbation theory, theories of
correlation functions that are based on the Ornstein-Zernike relation and the
mean spherical approximation, electrolytes, and recent work on ion channels in
biological membranes, where the competition between energy and size gives an
intuitively attractive explanation of the selectivity of cation channels.
Simulations of complex systems, including proteins in aqueous solution, should
be studied to determine the extent to which these concepts are useful for such
situations
