Chakravarthy Ayyagari

Multiscale Modeling of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer Micelles in Aqueous Solution

We present a multiscale modeling approach for simulation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer micelles in aqueous solution. We rely on systematic elimination of computationally expensive degrees of freedom yet retain implicitly their influence on the remaining degrees freedom in a coarser-grained model. Quantum chemistry (QC) calculations, atomistic explicit solvent (AES) molecular dynamics (MD) simulations, and coarse-grained implicit solvent (CGIS) simulations have been applied to investigate physical properties of these important self-assembling triblock copolymers. High-level QC calculations have been used to parametrize classical atomistic force fields that implicitly take into account and reproduce the important energetic and structural features due to correlations of electronic degrees of freedom. AES MD simulations utilizing the QC-based force fields have been used to provide structural and conformational properties of polymers in aqueous solution which were subsequently used for parametrization of the CGIS model using the Inverted Boltzmann method. The CGIS simulations were then employed to investigate structural properties of two PEO-PPO-PEO micelles (EO13-PO30-EO13 and EO99-PO65-EO99 also known as Pluronic L64 and F127, respectively) in aqueous solution.

Molecular dynamics simulations of HMX crystal polymorphs using a flexible molecule force field

Molecular dynamics simulations using a recently developed quantum chemistry-based atomistic force field [J. Phys. Chem. B, 103 (1999) 3570 ] were performed in order to obtain unit cell parameters, coefficients of thermal expansion, and heats of sublimation for the three pure crystal polymorphs of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The predictions for β-, α-, and δ-HMX showed good agreement with the available experimental data. For the case of β-HMX, anisotropic sound speeds were calculated from the molecular dynamics simulation-predicted elastic coefficients and compared with recent Impulsive Stimulated Light Scattering (ISLS) sound speed measurements. The level of agreement is encouraging.

Equilibrium sampling of self-associating polymer solutions: A parallel selective tempering approach

We present a novel simulation algorithm based on tempering a fraction of relaxation-limiting interactions to accelerate the process of obtaining uncorrelated equilibrium configurations of self-associating polymer solutions. This approach consists of tempering (turning off) the attractive interactions for a fraction of self-associating groups determined by a biasing field h. A number of independent configurations (replicas) with overlapping Hamiltonian distributions in the expanded (NVTh) ensemble with constant NVT but different biasing fields, forming a chain of Hamiltonians, were simulated in parallel with occasional attempts to exchange the replicas associated with adjacent fields. Each field had an associated distribution of tempered interactions, average fraction of tempered interactions, and structural decorrelation time. Tempering parameters (number of replicas, fields, and exchange frequencies) were chosen to obtain the highest efficiency in sampling equilibrium configurations of a self-association polymer solution based on short serial simulation runs and a statistical model. Depending on the strength of the relaxation-limiting interactions, system size, and thermodynamic conditions, the algorithm can be orders of magnitude more efficient than conventional canonical simulation and is superior to conventional temperature parallel tempering.