Getting Started


The simulations and visualizations in this tutorial were generated using Blender 2.70a and CellBlender 1.0. It may or may not work with other versions.

In order to quickly show what can be done with MCell and CellBlender, we'll create a simple model with a cube that has diffusing surface and volume molecules that react with each other to create new molecules. In the next section, we will visualize the molecules and plot the reaction data from within CellBlender.

Much of the theory will be skipped over, as it's available elsewhere. For a more detailed explanation of any given topic, please see the quick reference guide (pdf, html).

If you haven't already done so, install the required software listed in Required Software.

Starting Blender

Start Blender. If working from the command line, you can do this by opening a terminal and entering the following command:


After Blender starts, you should see a cube in the 3D View Editor. We will modify this object shortly, but first we need to organize our layout and set some project settings.


Organize Panels (Optional)

Hit the Scene button at the top of the Properties Editor.


There may be many Scene panels which are open (triangles pointing downward). In order to find things easily, take the time to collapse all of these panels by clicking the triangles next to their names. Alternatively, hit a while hovering over them.


Most Blender actions can be accomplished with either key commands or mouse clicks. But for speed, it is strongly advised that you learn the key commands.

A panel can also be moved around by clicking on the upper-right most portion of a panel (three diagonal lines) and dragging it around.


Let's move all the default Blender panels (Scene, Units, Keying Sets, etc.) down below the CellBlender panels. This will make finding everything much easier. When you're done, it should look like this:


We will also be working in the Object Context, so you may also want to follow the same procedure for it (collapsing default panels and moving CellBlender panels to the top). To navigate there, hit the cube button at the top of Properties Editor.


When you are done, switch back to the Scene context.


Once you are finished organizing the panels, hit File>Save Startup File. This will save your changes, so that every time you start a new blender project, it will already have this layout.

Set Preferences

Next, expand the Preferences panel by clicking the small triangle next to the name.


Hit the Set Path to MCell Binary button. Navigate to wherever you saved the MCell binary (e.g. /usr/local/bin/mcell on Linux) and select Set MCell Binary.

_images/mcell_file_navigate.png _images/mcell_set_button.png

A green checkmark should appear next to the file to indicate that it was successfully set.


Whenever you set this (or anything in Preferences), it will automatically be saved in a configuration file. Now, regardless of whether you are starting a new blend file or opening an existing one, it will already be set. You shouldn't have to change it again (unless you need to use a new version of MCell or Blender).

Set Project Directory

The project directory is set to be wherever the current blend file is saved. Let's save the file right now by hitting Ctrl-s, typing /home/user/mcell_tutorial/intro (where user is your user name) into the directory field and click Create New Directory when prompted. Type intro.blend into the file name field and hit the Save Blender File button.

Define a Surface Region

Now, we are going to create a surface region on the default Cube object, onto which we will later release molecules.

Hit the Object button at the top of the Properties Editor.


If you didn't organize your Object panels as suggested in Organize Panels (Optional), you'll need to scroll to the bottom of the Properties Editor panel. Expand the Define Surface Regions panel now.


You may have to scroll further if an action expands the size of a panel, because new fields may appear beyond the currently visible portion of the panel.

Hit the + button to add a new region, which appears in the list with the name "Region_0".


Rename this new surface region to "top" by changing the Region Name field from "Region_0" to "top" (again, you may need to scroll the panel to see all of the new fields created by the + button).


Assign a Surface Region

Move the cursor to the 3D View Editor. Hit Tab to change into Edit Mode (or enter Edit Mode via the Mode selection control). Hit Ctrl-t to triangulate the faces (or use Mesh>Faces>Triangulate Faces).


Hit Ctrl-Tab and select Face (or click on the "Face select" button) to enter face selection mode.


You can tell you're in Face Select mode (rather than Vertex Select or Edge Select modes) when each face has a square dot near its center.

Hit a (or use menu option Select>(De)select All) to deselect everything (edges turn black rather than orange). Then select just the top two triangular faces by holding Shift while right clicking on each of the two top faces to select them.


Under the Define Surface Regions panel, click the Assign button (remember that you might need to scroll down to see the new buttons added).


After clicking the Assign button, those top two faces will now have the "top" surface region assigned to them. They won't look any different, but they're now "tagged" with the name "top" which MCell can use to reference them.

We're done editing individual faces, so switch back to object mode by placing the cursor in the 3D View Editor and hitting the Tab key (or switch back to Object Mode via the Mode selection control below the 3D View Editor).

Adding to Model Objects List

Return to the Scene context by hitting the Scene button in the Properties Editor.


Then expand the Model Objects panel. With the Cube object still selected, hit the + button. This will add Cube to the list of mesh objects to be included in the MCell simulation. A green check mark will appear by the name to indicate that there are no problems with it.


Set Simulation Parameters

Find and expand the General Parameters panel. This is where we will define several parameters to be used throughout the project. Hit the + button six times, which will create six parameters named P1, P2, P3, P4, P5, and P6 each of which have 0 assigned to them by default.


Rename P1 to iters and put 1000 in the Expression field. We will do similar renamings and expression assignments to the other parameters.

  • Change P2 to dt and assign it 1e-5.
  • Change P3 to dc1 and assign it 1e-6.
  • Change P4 to dc2 and assign it dc1/10.
  • Change P5 to fwd_rc and assign it 1e8.
  • Change P6 to num_rel and assign it 2000.


Parameters can make use of basic mathematical operators using infix notation (e.g. P1+P2, P1/P2), certain keywords representing other mathematical operations (e.g. SQRT(P1), EXP(P1)), or be defined in terms of any previously defined parameter.

Now find and expand the Model Initialization panel. Change Iterations to iters, which we just defined in General Parameters. Change Time Step to dt. The units are in seconds.


Now, you might be wondering why you shouldn't directly assign numerical value here. You certainly could. However, it is generally good practice to define them as General Parameters, since it will be easier to update them if a given parameter appears in more than one location.

Define Molecules

We will now define three new molecule species. Expand the Define Molecules panel and hit the + button three times. This will create Molecule_1, Molecule_2, and Molecule_3.


Left click Molecule_1. Change the Molecule Name to vol1, the Molecule Type to Volume Molecule, and the Diffusion Constant to dc1. The units are cm2/second.


Repeat this process for the next molecule in the list, but call this one vol2. It will also be a Volume Molecule with a Diffusion Constant of dc1.


Now, change the third entry to surf1. The Molecule Type should be set to Surface Molecule and change the Diffusion Constant to dc2.


Define Reactions

In order to have our molecules interact with one another, we first need to define some reactions. Expand the Define Reactions panel and hit the + button. Change Reactants to vol1' + surf1,. Change Products to surf1, + vol2,. Be sure to use the commas and apostrophes shown in these examples.


Be especially careful about the apostrophe character since there are different "flavors" of single quotes. In particular, if you copy text from a web site and paste it into CellBlender (or an MDL file), be sure that you get the normal (ASCII 0x27) version. If you get an MCell error about "orientation not specified", then you might want to double check (and even retype) the single quotes in your reactions.

The meaning of these symbols will be explained in the Reaction Directionality section. Lastly, change Forward Rate to fwd_rc. The units are M-1s-1.


Create Release Sites

We have defined molecules and reactions, but we still need to release some molecules into our simulation.

Expand the Molecule Release/Placement panel and hit the + button twice. This will create two instances of a release site. Currently, they both have errors because we haven't assigned which type of molecule is going to be released.


Select the first instance (you may need to scroll down to see the entire panel), and change Site Name from Release_Site to vol1_rel.


Change Molecule to vol1.


Change Release Shape to Object/Region.


Change Object/Region to Cube.


Change Quantity to Release to num_rel. This will release 2000 vol1 molecules randomly throughout the interior of the Cube object.


Now select the second release site and make the following changes.

  • Change Site Name to surf1_rel.
  • Change Molecule to surf1.
  • Change Release Shape to Object/Region.
  • Change Object/Region to Cube[top].
  • Change Quantity to Release to num_rel.

This will release 2000 surf1 molecules randomly on the top surface region.

Create Reaction Output

Open the Reaction Output Settings panel. Click the + sign three times to save reaction data for each of the three different molecule species. They will show up as errors because they haven't been chosen yet, and there are no default values.


Select and highlight the first of the "Molecule name error" definitions and change its molecule name to "surf1" as shown here:


After clicking on "surf1", the top of that panel should look like this:


The description next to the check mark indicates that MCell will be counting and saving the count (number) of surf1 molecules in the World.

Repeat this process for the other two "Molecule name error" entries assigning them to vol1 and vol2 respectively to get this:


Create Visualization Output

Open the Visualization Output Settings panel (be sure to scroll down to see the whole panel):


Then click the Export All button to export all molecules for visualization:


Run Simulation

Expand the Run Simulation panel and change the End Seed to 2 and hit Run Simulation.


This will queue up two MCell simulations each with a different seed value. A green check mark will appear when the simulations have completed.



If you encounter any errors, you may need to check the command line for error messages. Windows users will have to select "Window>Toggle System Console".


Save Your Project

From the File menu, select the Save option (or hit Ctrl-s). This will save any changes that you have made to the blend, including the CellBlender specific settings (e.g. molecule definitions, release sites, etc).


  • We set up some project settings, including our path to MCell.
  • We started with Blender's default cube and divided it up into triangular faces.
  • The top faces were defined to be a special region that we called top.
  • We defined 3 different molecular species: surf1, vol1, and vol2.
  • We defined reactions with the surface molecules that transformed vol1 molecules inside the Cube into vol2 molecules outside the Cube.
  • We initialized the simulation by releasing specific numbers of molecules in (or on) specific regions of the physical model.
  • We specified which data to export, and we ran the simulation.