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\chapter{\label{chap:conclusion}CONCLUSION} |
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The preceding chapters and included appendices discuss the primary |
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aspects of the research I have performed and been involved with over |
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the last several years. Rather than presenting the topics in a |
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chronological fashion, they were arranged to form a series where the |
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later topics apply and extend the findings of the former topics. This |
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layout is more instructive and provides a more cohesive progression of |
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research efforts. |
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The first chapter laid out the foundation from which the research in |
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the later chapters is built upon, primarily the technique of molecular |
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dynamics. This chapter also introduces {\sc oopse}, the object |
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oriented parallel simulation engine, the unified code-base developed in |
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our lab for performing molecular simulations. Starting out as a |
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collection of separate programs written by different group members, |
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{\sc oopse} has developed into one of the few parallel molecular |
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dynamics packages capable of accurately integrating rigid bodies, |
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point multipoles, and metallic potentials.\cite{Meineke05} |
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The second chapter discussed correction techniques for handling the |
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long-ranged electrostatic interactions common in molecular |
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simulations, in particular our shifted-force ({\sc sf}) modification |
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of the damped shifted Coulombic summation method developed by Wolf |
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{\it et al.}\cite{Wolf99} In the work outlined here, we showed {\sc |
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sf} to be nearly equivalent to the more prevalent Ewald summation in |
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simulations of condensed phases, and since it is pairwise, it scales |
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as $\mathcal{O}(N)$ and lacks periodicity artifacts introduced through |
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heavy reliance on the reciprocal-space portion of the Ewald sum. We |
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extended the electrostatic damping technique used with {\sc sf} beyond |
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simple charge-charge interactions to include point-multipoles, and we |
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also identified optimal damping parameter settings to ensure proper |
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depiction of the dielectric behavior of molecular systems. Presenting |
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this technique early enables us to apply it in the systems discussed |
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in the later chapters and show how it can improve the quality of |
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various molecular simulations. |
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The third chapter focused on simple water models, specifically the |
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single-point soft sticky dipole (SSD) model for water. We implemented |
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this model and realized that we need to reparametrize it in order to |
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use it in our simulations. This lead to the development of SSD/RF and |
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SSD/E, new variants of the SSD model optimized for simulations with |
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and without a reaction field correction. These new single-point models |
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are more efficient than the common multi-point partial charge models |
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and better capture the dynamic properties of water. We also showed |
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that SSD/RF can be successfully used with damped {\sc sf} through our |
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multipolar extension of the technique. For the sake of completeness, |
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we also developed the two-point tetrahedrally restructured elongated |
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dipole (TRED) water model, which is optimized for use with the damped |
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{\sc sf} technique. Though there remain some algorithmic complexities |
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that need to be addressed (logic for neglecting charge-quadrupole |
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interactions between other TRED molecules) to use this model in |
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general simulations, it is approximately twice as efficient as the |
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commonly used three-point charge water models (i.e. TIP3P and |
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SPC/E). This work succeeds in extending the limits of the |
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computational efficiency of water models that can capture the |
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thermodynamic and dynamic properties of liquid water. |
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The final chapter deals with a unique polymorph of ice that we |
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discovered while performing water simulations with the fast simple |
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water models discussed in chapter~\ref{chap:water}. This form of ice, |
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which we called ``imaginary ice'' (Ice-$i$), has a low-density |
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structure which is different from any known polymorph from either |
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experiment or other simulations. The free energy analysis performed |
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here shows that this structure is in fact the thermodynamically |
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preferred form of ice for both the single-point and commonly used |
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multi-point water models under the chosen simulation conditions. We |
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then showed that inclusion of electrostatic corrections is necessary |
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to obtain more realistic results; however, the free energies of the |
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various polymorphs (both imaginary and real) in many of these models |
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was shown to be so similar that choice of system properties, like the |
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volume in $NVT$ simulations, will directly influence the expressed ice |
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polymorph. This work shows that researchers ought to be wary of using |
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these simplistic water models in the study of complex phase behavior |
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where the choice of a water model that includes many-body effects, |
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such as polarizability, might be more appropriate. |
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The work presented in this dissertation includes advancements in |
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simulation techniques, improved molecular models, and applications of |
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both in simulations of novel molecular systems. In addition to |
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answering interesting questions related to these topics, this work |
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opens up new routes which other researchers can utilize to extend and |
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improve their own work. Though specific in focus, through pathways |
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such as these, this work can gain wider utility and expand our |
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understanding of natural physical and chemical processes. |
