The Venerable Rotational Isomeric State Model

In the rotational isomeric state (RIS) approximation, each bond in the chain backbone is assumed to occur in any one of a small, discrete number of 

The average conformation of a polymer chain with an end-to-end distance r may be quantified by defining a general partition function Z r) 

Integration of Equation [17] over the momentum variable leads to 

Concentrating on the bond rotational partition function, we define for bond i  

Application of the RIS concept allows the replacement of the integral in Equation [21] by the following sum  

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