Method of pseudoaveraged functions in the RISM theory. Temperature dependence of oxytocin peptide hydration

In the temperature range from 280 K to 340 K oxytocin peptide is simulated by molecular dynamics in various surroundings including explicit water solvent. Using geometry configurations of the peptide we found Gibbs free energy by solving RISM equations for pseudoaveraged correlation functions. Various functionals of Gibbs free energy or excess chemical potential were compared. In the partial wave theory as well as in the approximation of Gaussian fluctuations Gibbs free energy is independent of temperature. Calculations by Singler–Chandler formula or its modifications taking account of repulsive bridge corrections show linear dependence of Gibbs energy on temperature. Analysis suggests that the temperature dependence is associated with the term proportional to the square of the total correlation function.