New Insight into the Stability of CaCO3 Surfaces and Nanoparticles via Molecular Simulation

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Submitted by dquigley

Feb. 10, 2022, 4:59 p.m.

New Insight into the Stability of CaCO3 Surfaces and Nanoparticles via Molecular Simulation

A. Matthew Bano, P. Mark Rodger, and David Quigley
Langmuir 2014, 30, 25, 7513–7521
DOI:  10.1021/la501409j          

Brief Description
This is a molecular simulation study on the energetics of calcium carbonate nanoparticles and surfaces. It uses the LAMMPS simulation package to simulate a popular model for the interactions between calcium ions, carbonate ions, and water.

A particular conclusion is that the energy of interaction between many of the surfaces studies and water is overall negative, i.e. based purely on energetic arguments these crystals are unstable and should spontaneously cleave in the presence of water. It is shown that this conclusion can be reversed by including a rough estimate of the surface entropy.

The LAMMPS package is open source, and so it should be possible to reproduce the surface energy data using the provided input structures and the force field parameters specified in the sources cited.
Why should we reproduce your paper?
The negative surface enthalpies in figure 5 are surprising. At least one group has attempted to reproduce these using a different code and obtained positive enthalpies. This was attributed to the inability of that code to independently relax the three simulation cell vectors resulting in an unphysical water density. This demonstrates how sensitive these results can be to the particular implementation of simulation algorithms in different codes. Similarly the force field used is now very popular. Its functional form and full set of parameters can be found in the literature. However differences in how different simulation codes implement truncation, electrostatics etc can lead to significant difference in results such as these. It would be a valuable exercise to establish if exactly the same force field as that used here can be reproduced from only its specification in the literature. The interfacial energies of interest should be reproducible with simulations on modest numbers of processors (a few dozen) with run times for each being 1-2 days. Each surface is an independent calculation and so these can be run concurrently during the ReproHack.
What should reviewers focus on?
Table 1 (crystal-vacuum) and figure 5 (crystal-water) present the surface/interface energies of interest. The latter will need to be calculated from ensemble-averaged energies over simulations of modest length. Error bars should be computed for comparison. In the first instance I'd suggest reproducing these using LAMMPS to establish if the force field and simulation protocol can be reproduced from the data given in the paper and its references (and with the current version of LAMMPS). Input structures are available via GitHub. As a follow-up (given time) it would be valuable to establish if the same surface enthalpies can be reproduced by implementing the same force field and simulation protocol in a different package, such as GROMACS or DL_POLY. The nanoparticle results within this paper should also be reproducible but will require longer run times than can likely be accommodated in the ReproHack event.


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