Revisiting the work "Brownian motion with time-dependent friction and single-particle dynamics in liquids" by Lad, Patel, and Pratap [Phys. Rev. E 105, 064107 (2022)]

Recently, Lad, Patel, and Pratap (LP&P) [Phys. Rev. E 105, 064107 (2022)] revisited a microscopic theory of molecular motion in liquids, proposed by Glass and Rice [Phys. Rev. 176, 239 (1968)]. Coming from this theory, LP&P derived a new equation of motion for the velocity autocorrelation function (VAF) and argued that the friction coefficient of particles in liquids should exponentially depend on time. The numerical solution of this equation was fitted to the results of molecular dynamics simulations on different liquids. In our Comment [Phys. Rev. E 108, 036107 (2023)], we showed that this solution, obtained under the condition of zero derivative of the VAF at time t = 0, is physically incorrect. This was evidenced by our exact analytical solution for the VAF, not found by LP&P, and numerically, by using the same method as in the commented work. In the Reply [Phys. Rev. E 108, 036108 (2023)], Lad, Patel, Pratap, and Pandya claimed that our solution does not satisfy all the necessary boundary conditions and is thus not appropriate for the description of atomic dynamics in liquids. Until and unless proven otherwise they do not find any reason for the reconsideration of their theory. Here we give a rebuttal to this Reply and, returning to the original work by LP&P, show that the presented there equation for the VAF is wrong. Due to errors in its derivation, it is, among other inconsistencies, incompatible precisely with the boundary conditions for the VAF which lie in the basis of their theory.Comment: 10 pages, 2 figure

Dynamics of Rouse polymers in Maxwell fluids

A generalization of the Rouse model of the dynamics of polymers in solution is proposed. The motion of long polymer chains is considered to be due to exponentially correlated random forces driving the polymer segments, which is a more realistic model than the approximation of white thermal noise in the standard theory. Due to the fluctuationdissipation theorem such a model is consistent with the assumption that the solvent has weakly viscoelastic properties, which corresponds to the theory, originally proposed by Maxwell and later substantiated coming from first principles. A consequence of such approach is the appearance of “memory” in the polymer dynamics. To obtain a correct description at short times, we also include inertial effects into the consideration. Discrete and continuum models of the universal dynamics of polymer chains are built. Exact solutions are obtained for the center of mass motion of the polymer coil in the discrete variant of the theory. The time correlation functions describing the dynamics of internal modes are calculated in the continuum approximation. The results significantly differ from those in the standard Rouse theory and its later generalizations valid at long times