Date of Award
2026
Degree Type
Thesis
Degree Name
Master of Science in Chemical Engineering (MSChE)
Department
Chemical, Biomolecular, and Materials Engineering
First Advisor
Michael L. Greenfield
Abstract
This thesis investigates whether the selection of thermodynamic ensemble introduces quantifiable differences in the stress relaxation modulus G(t), storage modulus G”(w), loss modulus G”(w), and phase angle d(w) computed from equilibrium molecular dynamics simulations of the AAA-1 asphalt model. A 72-molecule, 12-component representation of AAA-1 bitumen was simulated under three ensembles (NPT, NVT, and NVE) at three temperatures (400.15 K, 443.15 K, and 533.15 K) with three independent replications at each condition, yielding 27 total simulations. Stress autocorrelation functions were computed from each production trajectory and Fourier-transformed to obtain frequency-dependent rheological properties.
The computed properties show no meaningful dependence on ensemble selection. The stress relaxation modulus and frequency-dependent moduli overlap closely across NPT, NVT, and NVE ensembles at each temperature, indicating that thermostat and barostat coupling do not introduce systematic biases into the calculated mechanical response. A pronounced peak in the phase angle from 0.001-0.04 rad/fs, previously observed in NVT simulations and hypothesized to arise from Nosé-Hoover thermostat coupling, appears with equal magnitude across all three ensembles including NVE, ruling out ensemble-specific artifacts as its origin. The feature corresponds to periods of approximately 150-6300 fs, consistent with molecular angular and torsional vibration timescales, and its origin remains a subject for future investigation. An FFT-based approach to computing the stress auto-correlation function made time sampling of stress components at 1 fs resolution tractable, reducing the number of required calculations by approximately six orders of magnitude. Time-temperature superposition was applied successfully to the complex modulus, yielding shift factors spanning two orders of magnitude across the simulated temperature range.
Recommended Citation
Hyde, Mason E., "ENSEMBLE DEPENDENCE OF MECHANICAL RELAXATION IN MOLECULAR DYNAMICS SIMULATION" (2026). Open Access Master's Theses. Paper 2725.
https://digitalcommons.uri.edu/theses/2725