RMCProfile – III

  • Exercise files are found here

    Introduction

    In this tutorial, we will show how to use GSAS-II in conjunction with RMCProfile (“RMCProfile: Reverse Monte Carlo for polycrystalline materials”, M.G. Tucker, D.A. Keen, M.T. Dove, A.L. Goodwin and Q. Hui, (2007) Jour. Phys.: Cond. Matter, 19, 335218. doi: https://doi.org/10.1088/0953-8984/19/33/335218) to do a combined fit with neutron and x-ray powder diffraction data and pair distribution functions (PDF) created from them with a “big box” disordered model.

     

    Before getting started you must obtain the most recent Version 6 of the RMCProfile executable (at this writing this is RMCProfile V6.7.6 Beta Serial). Obtain this from www.rmcprofile.org by following the prompts from Downloads. This version is available for Windows, Mac OSX and Linux. The download will be a zip file; save it somewhere convenient. Pull from it only the RMCProfile_package/exe/rmcprofile.exe and place it in the GSASII main directory or in a new subdirectory (e.g. GSASII/RMCProfile); no other files are needed although you may wish to also retrieve the contents of the tutorial subdirectory. It contains rmcprofilemanual.pdf and rmcprofile_tutorial.pdf which may be of interest since the GSAS-II/RMCProfile tutorials are based on the latter.

     

    The process for using RMCProfile begins with a normal Rietveld refinement of the average structure. For the kind of disordered materials of interest here, this may give bond lengths that are frequently too short for the atoms involved and sometimes extreme apparent thermal motion for some of the atoms. These effects arise because the average structure places these atoms too close to one another; a more localized view would show coordinated atom displacements or reorientation of groups of atoms to avoid the close contacts. RMCProfile is then used to characterize these local displacements by fitting to the diffraction data and its Fourier transform as a PDF.

     

    The material used in this tutorial is the same as described in the RMCProfile tutorial Exercise 6, gallium phosphate (GaPO4). GaPO4 is a piezoelectric crystal of corner sharing GaO4 and PO4 tetrahedra arranged as in a doubled c-axis α-quartz structure.  The average structure determined by Rietveld refinement results in Ga-O and P-O bond lengths that are shorter than implied by the corresponding peak positions in either an x-ray or neutron PDF. The X-ray data used here was obtained at room temperature on beam line I15 at the Diamond light source, and the neutron data on the POLARIS diffractometer at ISIS, Rutherford-Appleton Laboratory, Harwell Campus, UK. The objective here is to use RMCProfile to model the structural distortions that yield the PDF derived bond lengths.

     

    If you have not done so already, start GSAS-II.

    Rietveld Refinement of Gallium phosphate (GaPO4):

    Step 1: Read in the data file

    1. Use the Import/Powder Data/from GSAS powder data file menu item to read the data file into the current GSAS-II project. This read option is set to read any of the powder data formats defined for GSAS (angles in centidegrees, TOF in µsec). Other submenu items will read the cif format or the xye format (angles in degrees) used by topas, etc.  In those cases, you would change the file type to cif format or the xye format to see them. Because you used the Help/Download tutorial menu entry to open this page and downloaded the exercise files (recommended), then the RMCProfile-III/data/... entry will bring you to the location where the files have been downloaded. (It is also possible to download them manually from https://subversion.xray.aps.anl.gov/pyGSAS/Tutorials/RMCProfile-III/data/. In this case you will need to navigate to the download location manually.)

    For this tutorial you should see the data file in the file browser, but if extensions on data files are not the expected ones, you may need to change the file type to All files (*.*) to find the desired file.

    2. Select the 105870-pe-GaPO4_ed.gsas data file in the first dialog and press Open. There will be a dialog box asking Is this the file you want? Press Yes button to proceed.

    Next will be a apparently empty file selection dialog for the instrument parameter file; switch the file type to GSAS iparm file (*.prm,…). Then one (inst_i15_pdf_april14.prm) is shown. Select it; the pattern will be read in and displayed

    Step 2. Set limits

    Since we want to avoid the abrupt jump and (as it turns out) low angle contaminating peaks, we want to set the lower limit to just below the first large visible Bragg peak and the upper limit needs to be set to include just the range with Bragg intensities (~0.4Å); go the Limits entry in the GSAS-II data tree.

    Change the 1st entry to 2 and the second to 25; the plot will change by moving the green and red dashed lines.

    Step 3. Import GaPO4 phase

    The average structure of GaPO4 is that of a doubled c-axis α-quartz. Space group P3121, a=4.90063 c=11.04809 with 4 atom positions. We can import it from an old gsas experiment file. Use the Import/Phase/from GSAS .EXP file menu item to read the phase information for GaPO4 into the current GSAS-II project. Other submenu items will read phase information in other formats.  Because you used the Help/Download tutorial menu entry to open this page and downloaded the exercise files (recommended), then the RMCProfile-III/data/... entry will bring you to the location where the files have been downloaded. (It is also possible to download them manually from https://subversion.xray.aps.anl.gov/pyGSAS/Tutorials/RMCProfile-III/data/. In this case you will need to navigate to the download location manually.) Select the GAPO4_XRAY_BAK.EXP file (only one there) There will be a dialog box asking Is this the file you want? Press Yes button to proceed. You will get the opportunity to change the phase name next (it is very long, I changed it to GaPO4-X; this will be used as the file root for many of the files produced by RMCProfile. NB: because of restrictions in RMCProfile it is important that the phase name not have any spaces); press OK to continue. Next is the histogram selection window; this connects the phase to the data so it can be used in subsequent calculations.

    Select the histogram (or press Set All) and press OK. The General tab for the phase is shown next

    Step 4. Check atom positions

    It is usually wise particularly for trigonal/hexagonal structures, to check the atom positions to be sure atoms are properly placed on special positions. Select the Atoms tab

    In this case the z-position for both Ga and P are not sufficiently precise for use in RMCProfie. The Ga atom is at z=1/3 and the P atom is at z=5/6. Enter the fractions into the respective positions in the table; GSAS-II will do the math to the full double precision available in python. You should get

    If this isn’t done then some of the input to RMCProfile will not be correctly prepared.

    Step 5. Set the background

    Although there is not much diffuse scattering, the default background will be insufficient for reasonable fitting. Select the Background item from the GSAS-II data tree

    Be sure that “chebyschev-1” is used for the background function; RMCProfile only recognizes that form to compute the background for the diffraction pattern. Select 6 for the number of terms.

    Step 6. Rietveld refinement

    We are now ready for the first Rietveld refinement; select Calculate/Refine from the main GSAS-II menu. Before it starts a file selection dialog will appear; select a file name (no extension). I chose “GaPO4”; do not use the phase name for this purpose because RMCProfile will use that as the root for the many files it creates. Also, you should create a new directory for this exercise while in this dialog box; it will be rapidly filled up with RMCProfile files which can lead to considerable confusion if mixed in with other files. For Windows, after navigating to a suitable location, a new directory can be made by a right mouse click and selecting “New/Folder” from the popup menu. It will appear with the name “New Folder”; change the name and select it (it will be empty). Other operating systems will have similar methods. Finally press OK to save the GSAS-II project (GaPO4.gpx) and start the refinement. It will quickly converge to give a reasonable Rwp (~9.8%)

    I pressed the ‘s’ key so that sqrt(intensity) shows on the y-axis; makes things a bit easier to see. It is evident that the lattice parameter as well as the sample broadening terms needs adjusting. Go to the General tab for the GaPO4-X phase and select Refine unit cell. Then select the Data tab and select size and microstrain for refinement. (A word to the wise; set the starting value for size to 0.1 μm). Then do Calculate/Refine from the main menu. The fit will greatly improve (Rwp ~5.25%)

     

    To finish up add the following parameters:

    GaPO4-X/Atoms: XU for all four atoms (GSAS-II recognizes that the Ga & P atoms are fixed in y & z positions).

    Do Calculate/Refine from the main menu; the fit will scarcely change (Rwp~5.25%).

    Step 7. Draw structure

    Select the Phases/GaPO4-X/Draw Atoms tab; the list will be shown and the four unique atoms drawn on the plot.

    To improve this do the following:

    1) Double click the empty upper left box in the Draw Atoms table; all atoms will turn green and the four table rows will be grey (or blue).

    2) Under the menu Edit Figure select Fill unit cell; the figure will be redrawn showing all atoms in the cell.

    3) Double click the Type column heading and select the Ga and P atom types

    4) Under the menu Edit Figure select Fill CN-sphere; the figure will be redrawn with four O atoms about all Ga and P atoms.

    Next we will explore this with a RMCProfile simulation. To keep this drawing, you may want to save the GSAS-II project.

    Reverse Monte Carlo Simulation of GaPO4

    RMCProfile is most effective if a large box is used for the modelling; this requires very long running times (10-20 hrs for GaPO4) before a meaningful result is obtained. However, for the purposes of this tutorial, we will be using a smaller box model that converges in a more reasonable time (~10min). The result will clearly fit the data but the model is too small to give enough atom displacements to be meaningful, however this exercise will show you how to set up RMCProfile from within GSAS-II.

    To start select the Phases/GaPO4-X/RMC tab; if rmcprofile.exe is within the GSASII directory the data window will show

     

    At the top is a radio button selection for RMCProfile and fullrmc. The latter is an alternative big box modeling program (not working – under development); RMCProfile is selected by default and all below are its setup controls. There are four major sections (metadata, major controls, restraints & constraints and data controls); we will work through each of these in turn. The information you enter here is retained in the GSAS-II project file so you can easily try alternative setups without having to enter everything over again.

    Step 1. Set metadata items

    The entries here are for your convenience; there is no explicit use made of any of them, but they will appear in some of the output files from RMCProfile and of course will show here on subsequent views of this window. Fill out as many of them as you see fit. I entered some things for each as the defaults are somewhat nonsensical.

    Step 2. Set general controls

    The running time is defaulted to 10 minutes with a Save interval of 1 minute. At each save time a number of files are written by RMCProfile; these can be viewed by using the Operations/View command (more about this later – don’t bother trying it now, there is nothing to see). For the purposes of this tutorial leave them at their defaults.

    The big box model used by RMCProfile is described as multiples of the unit cell axes. In this case we want a 6x6x4 box so enter 6 for each of X-axis, Y-axis and 4 for Z-axis.

    Next is to set the order of the atom types in the structure; this is used to construct atom-atom distance restraints on the modelling. The order here (Ga, P, O) is appropriate; if changed the window will repaint updating various entries possibly resetting some entries to defaults. I suggest you decide the order now and then don’t change it later. Set the maximum shift for Ga & P to be 0.05 (the value for O is OK). When done the window should look like

    Skip the next item (Atom swap probabilities). This is for cases in which atoms can exchange sites.

    Step 3. Set constraints/restraints

    Next is to set the “Hard” minimum atom-atom separation and the allowed search rage for each of 6 pairs of atom types. Possible values for this table can be obtained in GSAS-II by computing bond lengths. Select the Atoms tab and then select all atoms by double clicking on the empty upper left box in the table.

    Then do Compute/Show distances & angles from the menu; a popup will appear

    Set the Bond search factor to 1.5 and the Angle search factor to 0.0 (to skip angle calculations), and press Ok. The console will show a very long table showing all interatomic distances out to 4-5Å. Find the shortest distance for each of the 6 types of atom pairs in the GaPO4-X/RMC data tab. For each bond subtract 0.2Å or 0.4 Å for non-bonds and enter the value as Hard min and Search from. Then add 0.2 Å or 0.4 Å and enter that in the to row. You want to straddle the Rietveld determined values by enough to encompass possible distances so examine the distance table for the full possible ranges for each atom-atom distance so the search bands may be even wider than +/- 0.4 Å. When done the window should look like

    Scroll down to the bottom of the window for the last section.

    Step 4. Select data to be fitted

    We will be using both x-ray and neutron data for GaPO4 for fitting by RMCProfile. The first step is the selection of the x-ray powder pattern (“Bragg”) for processing. This is taken from the PWDR entry in the GSAS-II project and accessed from the pulldown; there is only one “PWDR 105870-pe-GaPO4_ed.gsas Bank 1” that was used earlier in your Rietveld refinement of GaPO4. If you had used multiple banks in the Rietveld refinement, all would be shown in the pulldown, but only one can be selected. Set the weight to 100 (NB: smaller numbers means a heavier weight).

    Next press the Select button for the “Neutron real space data; G(R)” line; a FileDialog should appear. Because you used the Help/Download tutorial menu entry to open this page and downloaded the exercise files (recommended), then the RMCProfile-III/data/... entry will bring you to the location where the files have been downloaded. (It is also possible to download them manually from https://subversion.xray.aps.anl.gov/pyGSAS/Tutorials/RMCProfile-III/data/. In this case you will need to navigate to the download location manually.)

    The FileDialog should show

    Select gapo4_600uamps_rmc_35.gr; it will be copied from this location to your working directory for this tutorial (RMCProfile requires all its files to be local). The window will be redrawn showing the new entry; the weight is fine.

     

    Next press the Select button for “Neutron reciprocal space data: F(Q)”; the FileDialog will show again. Select gapo4_600uamps_rmc_35.fq; again it will be copied to your working directory. The window will be redrawn; change the weight to 0.01.

    Finally press the Select button for “Xray reciprocal space data: F(Q)”; )”; the FileDialog will show again. Select gapo4_xray_rmc.fq; again it will be copied to your working directory. The window will be redrawn; set the weight to 0.1. You should also check the Fit scale factors? box. The window should show

    when done.

    Step 5. Setup RMCProfile files

    You are now ready to setup the RMCProfile input files.. Do Operations/Setup RMC from the menu; the console should report that files were written

    and your local directory should now have 9 files

    These text files contain data needed by RMCProfile for the fitting of GaPO4. They are:

    GaPO4-X.back- the 6 coefficients for the chebyschev-1 function needed to compute the background for the Bragg pattern

    GaPO4-X.bragg – the powder pattern used in the Rietveld refinement

    GaPO4-X.dat – the RMCProfile controls file; described in full in the RMCProfile User Manual. It can be edited if need be, but remember it is rewritten each time Operations/Setup RMC is done.

    GaPO4-X.inst – the instrument parameter coefficients for the x-ray peak shape function used by GSAS-II and implemented in RMCProfile for computing the Bragg pattern.

    GaPO4-X.rmc6f – the big box set of atom positions. It is normally not rewritten when Operations/Setup RMC is done unless the X-axis, Y-axis, or Z-axis lattice multipliers are changed. Most important is that it will contain the set of big box atom positions as updated by RMCProfile according to the Save interval (every 1 min in this case).

    You are now ready to run RMCProfile.

    Step 6. Run RMCProfile

    To run RMCProfile from inside GSAS-II, do Operations/Execute. You will first see a “nag” note asking you to cite the publication describing RMCProfile; press OK. The program will start in a new console window – processing will initially be pretty fast for this case and then slow down as the modelling proceeds. It reports Time used and Last saved. Once the latter is nonzero you can view intermediate results to see its progress.

    I have captured the initial RMCProfile console output (done by simply clicking anywhere in the window – note the white square. This temporarily stops processing; it can be restarted by pressing any key with the focus on the window). Things to look at are the several chi^2 values; there will be an overall one and one for each of the 5 data sets used in the fitting. The last one “X-ray real-space” is generated within RMCProfile by doing the Fourier transform from the F(Q) input data to form the x-ray g(R) distribution (e.g. the x-ray PDF). Since we’ve set the Fit scale factors option; the “Renorm” values are other than unity (the Expt 1, especially). These will approach 1.0 as the modelling continues. Note that you can exit GSAS-II and RMCProfile will continue running. When it finished the console shows (in part)

    The chi^2 values are now close to unity and not very different from each other; this is what is desired.

     

    After RMCProfile finishes note that the project directory now has ~35 files many of which are just temporary ones created by RMCProfile. We will be looking at just the *.csv files and the GaPO4.rmcf6 file; the latter contains the last atom configuration acceptable by RMCProfile thus representing a best disordered model.

    A note about weights:

    The choice of weights for RMCProfile may seem to be arbitrary in these exercises, but I have selected them so that each of the chi^2 values converge to be roughly equal and somewhere just above unity. This selection generally gives the best performance in the reverse Monte Carlo calculation and requires some “trial & effort” in selecting the values. If one of the chi^2 values is too large (>>1, i.e. weight too small) at convergence, then adjustments in the structure will have been dominated by moves that lower that chi^2 sometimes at the expense of others. Conversely, if one chi^2 is too small (>>1, i.e. weight too large) then moves will tend to satisfy the others and there will be little sensitivity to that data set. In this case my chosen weights have yielded chi^2 values between 0.6 and 1.6; this is essentially ideal. If they vary by more than an order of magnitude or are much less than unity, you should adjust the weights.

    Step 7. Viewing results from RMCProfile

    Do Operations/View; a FileDialog showing only *.csv files will appear

    They all should have the same prefix in their name, “GaPO4-X” which is the phase name from the GSAS-II project file. Select any one of them – it doesn’t matter. All of them will be read and their contents displayed as individual plots. For example the chi^2 plot shows the progress of the RMCProfile fit

    If you zoom into the end of the run, you can see that RMCProfile is still improving the fit slowly even after 10 minutes of running.

     

    Comparing the Bragg plot with the PWDR plot shows that the RMCProfile fit is very similar to the Rietveld fit

    This plot was made by dragging one plot tab to the bottom of the screen to create the second frame.

    The neutron G(R) partials plot shows the identity of each feature; for example, the first 2 sharp peaks arise from P-O (1.53Å) and Ga-O (1.73Å) distances.

     

    Of special note are the sharp edges on some of the peaks; these indicate that the parameter ranges we used in Step 3 above were too restrictive. We should adjust these and rerun RMCProfile. NB: if this plot also shows sharp peaks in the Ga-Ga partial curve, you may have not done the atom check in Step 4 of the Rietveld refinement. Not setting the atom positions correctly gives duplicate atom positions which then aren’t moved by RMCProfile. You’ll need to go back to that step and then repeat everything after that.

    Step 8. Adjustment of parameter ranges

    Select the GaPO4-X/RMC tab (if it wasn’t selected before) and find the constraints & restraints section

    Referring to the G(R) partials plot above, we need to use smaller P-O and Ga-O hard minima and larger P-O and O-O limits. I chose 1.2 for both P-O and Ga-O minima, 2.0 for the P-O maximum and 5 for the O-O maximum. Don’t forget to change both Hard min & Search from values. When done the window shows

    Next do Operations/Setup RMC; this will rewrite all but the rmc6f file thus keeping the model attained in the 1st RMCProfile run. Then do Operations/Execute and press Ok on the nag note. RMCProfile will run for another 10 min. When done do Operations/View and examine the G(R) partials plot

    It is better, but the O-O partial still shows an abrupt cutoff (now at 5A). We can remove the O-O distance restriction altogether by setting the maximum to zero. Again do Operations/Setup RMC and then Operations/Execute to rerun RMCProfile with the new setting. When done (another 10 min) do Operations/View to see the results.

    The sharp edges are gone so there are no artificial limits imposed on the model that restrict the model unnecessarily.

    A comparison of the neutron G(R) (top) and the x-ray g(R) (bottom) show reasonable agreement between the RMCProfile model and the data – probably could be better with a larger box model (and many hours of computer time!).

    Step 9. View the big box structural model for GaPO4

    Next, we can view the resulting big box structure. Do Import/Phase/from RMCProfile .rmc6f file; a FileDialog box with one entry will show the required file, GaPO4-X.rmc6f. You will first see a Is this the file you want popup window; select Yes. Next will be an Edit phase name popup. The proposed name is the same as the existing one; GSAS-II will rename this one by adding ‘_1’ to the end. Next will be a popup for Add histograms; respond Cancel because you don’t want this phase to be in any subsequent Rietveld refinement. Looking at the General tab for this phase we see it is 6X in a & b and 4X in c axes relative to the original and has no symmetry (space group P 1).

    Select the Draw Atoms tab for this phase; a van der Waals ball model will be drawn. You can select the Ga & P atoms & fill the CN-sphere for them (does take a long time, >10min – be patient) and then change all the atoms to ball and stick style. The CN-sphere filling works because the RMCProfile structure still has translational symmetry across its 6x6x4 lattice. You should see something like

    Notice the apparent tilting the PO4 and GaO4 tetrahedra. The stray O atoms are bound to PO4 and GaO4 tetrahedra in neighboring boxes.

     

    NB: this import facility can be used to load any rmc6f file, e.g. from a RMCProfile run done outside of GSAS-II; all the plotting and tools in this and the following steps are available for these big box models.

    Step 10. View the disordered average structure

    We can compress the big box result back into the original average crystal structure to see how the disordered sites compare with the average ones. Select the General tab for the big box phase (SF6_1) and then do Compute/Transform. A popup dialog will appear

    This is the general tool inside GSAS-II for all sorts of structure transformations. Here we will use it to push all the big box atoms back into the average structure unit cell. Recalling that the big box model is 6x6x4 the original unit cell, we simply want to reverse the process. Enter 1/6, 1/6, 1/4 into the diagonal elements of the M matrix. The GUI will convert them to the decimal equivalent (0.167,…). The target space group should be P 1. You should see

    Press Ok to do the transformation. A new phase, “GaPO4-X_1 abc” will be formed as triclinic with cell axes that match the original hexagonal ones. Select Draw Atoms; a van der Waals model will appear.

    Select the Draw Options tab and reduce the van der Waals scale to about 0.05.

    This compares pretty well with the original structure with the ellipsoids drawn at 99% probability.

    In particular the O atoms are more widely distributed in space than the P or Ga atoms.

     

    Step 11. Polyhedral comparison for the big box model

    In this step we will examine how the suite of PO4 and GaO4 tetrahedra deviate from an ideal tetrahedron. This analysis is currently restricted to structures with P1 space group symmetry; this is what we have from Step 9 above. Select Phases/GaPO4-X_1 from the data tree; the General tab will appear.

    What we want is at the bottom of this panel; scroll down to the bottom

    At the very bottom is the control for polyhedral comparison. All that is needed is to select the central atom (“origin atom type”) and the polyhedral vertices (“target atom type”). Select P for the former and O for the latter. Then do Compute/Compare from the menu; a popup for setting Distance Angle Controls will appear

    In some cases the bond search factor may need to be adjusted to get the right number of vertices for the polyhedra; in this case 0.85 is fine (NB: in this use the angle ranges are ignored). Press OK; a progress bar will show the atoms being processed. The console may show some atoms being skipped because the number of vertices wasn’t either 4 (tetrahedron) or 6 (octahedron); all but one are ok here. Because there are 432 P atoms to be looked at in this big box model, this calculation can take a number of minutes. The General tab reappears already scrolled to the bottom

    A new button (Show Plots?) has appeared after the atom selections; press it. A number of new entries will appear on the plot window; we will discuss each in turn. Select the Tet P-O Bond for GaPO4-X_1 tab

    This gives the distribution of P-O bond lengths for all the tetrahedra found across the big box model. It is rough because of the small size of the model, but there is a definite asymmetry in the P-O bond distribution. The console will have listed the average value with the standard uncertainty in parentheses.

    Next select the Tet P-O Tilts for GaPO4-X_1 tab and the Tet P-O tilts for GaPO4-X_1 tab (NB: slight difference in name!). I’ve put then side-by-side.

    This gives the distribution of axial tilts of the PO4 molecules from the reference tetrahedron (aligned so that the  axes are along the Cartesian axes) on the left and the rotation vectors for the tilts colored by the tilt angle on the right. There is an apparent split distribution centered at ~48 deg and ~68 deg (the rather meaningless in this case average tilt is also shown on the console); the 48 deg set are about 2 axes along (1,-1,0) and (-1,1,0) while the 68 deg set is roughly about the (0,0,-1) axis. This probably arises from the algorithm selecting different legs of the tetrahedron for calculating the tilt and actually represents the same rotation.

    Next select X-Delta, Y-Delta and Z-Delta tabs in turn.

    I have made this plot by dragging the respective plot tabs to one of the right (or left) edges to create a 3 pane plot. These show the displacement of the O atoms from the ideal tetrahedral vertices along each Cartesian axis. Here they are arranged about the zeros fairly tightly. If there were structural distortions to the tetrahedra (e.g. Jahn-Teller distortions) these could show in these plots.

    To do the same thing for Ga-O polyhedra, select Ga instead of P for the origin type atom and then do Compute/Compare. Again there will be the Distance Angle Controls popup; no changes needed. The calculation will again take several minutes to complete. When done press the Show plots? button; more plots will be added to the plot window. I show both sets of tetrahedral bonds as a stacked plot; the Ga-O bonds are also skewed but to the shorter side as opposed to the P-O bonds.

    Similar comparisons can be made with the other plots. NB: if the project is now saved all the data needed to reproduce the plots is also saved; any of them can be seen without redoing the Compare calculation.

     

    This completes this RMCProfile tutorial; you should save the project.

    Final note

    A production run with enough atoms to give decent statistics would be for a box that is perhaps 12x12x8 the original unit cell and would require a 10-20 hour run time. It would contain several thousand atoms instead of ~2600 as used here.