Graduation Year

2020

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Chemistry

Major Professor

Brian Space, Ph.D.

Committee Member

Arjan van der Vaart, Ph.D.

Committee Member

Shengqian Ma, Ph.D.

Committee Member

Preston Moore, Ph.D.

Keywords

molecular dynamics, electronic structure, gas sorption, Monte Carlo, porous materials, potential energy

Abstract

The energy used in the separation and purification of small volatile atoms and compounds represents a sizablefraction of the world’s energy consumption as a whole 1 Noble gases in particular represent a unique challenge as their separation currently relies on distillation methods that work by virtue of small differences in the density of their pure phases. Highly porous Metal-Organic Materials (MOMs) offer a cheaper, more efficient alternative to cryogenic distillation, and studying their properties using various simulation methods has become an important and active area of research. The family of Potentials with “High Accuracy, Speed and Transferability” (PHAST) have been demonstrated to be unphysically repulsive at short separation distances due to the limitations of the Lennard-Jones 12-6 function. The updated PHAST model, herein “PHAHST” has evolved to use a more fundamentally grounded repulsion/dispersion potential form, including exponential repulsion, C 6 , C 8 , and C 10 dispersion parameters and dispersion-repulsion damping functions. This new functional form, herein PHAHST, is of the Tang-Toennies type, and is shown to excel at modeling sorbates in heterogeneous environments such as those encountered within nanoporous media 2 This improvement to the potentials reduces the total number of sites of previous models, offering less cumbersome computational cost when using them. Modeling polarization through the use of induced dipoles and representing the permanent electrostatics as point charges is retained. The choice of parameter mixing rules has also been updated, retaining the use of arithmetic averaging for sigma, but adopting a more sophisticated rule for epsilon. Verification and validation of the new model is performed on small (N≤7) rare-gas clusters composed of helium and neon atoms. High level ab initio calculations (CCSD(T) using aug-cc-pVTZ/QZ basis functions with extrapolation to the complete basis set limit) are employed to calculate the energy profile of an incident adsorbing particle. Many-body van der Waals (vdW) effects are compared, including Axilrod-Teller-Muto (AT) and coupled-dipole.

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