The intermolecular rotational potential energies for poly(dimethylsiloxane) (PDMS) chains aredirectly obtained from a priori probability Pαβ. Here the differing statistical weight matrices for the Si-Oand O-Si bonds are considered in calculating the configuration partition function. In the Bahar's model

asthe same statistical weight matrices for the Si -O and O- Si bonds are adopted

there exists a large deviationof α priori probability Pαβ between the theory and the molecular dynamics (MD) simulation. Our model gives satisfactory agreement with experiment on the mean-square unperturbed end-to-end distance

the mean-square dipole moment and its temperature dependence

and the molar cyclization equilibrium constants for dimethylsiloxane oligomers.This new rotational isomeric state approach can be widely applied to other chains

such as —CH2—C[(CH2)mH]2—and —O—Si(CH2)mH]2 for arbitrary m.