# Gmsh and only C3D10 elements

**URL:** <https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859>\
**Category:** Tool interfaces\
**Created:** [August 23, 2025, 4:49pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859 "2025-08-23T16:49:53Z")\
**Posts on this page:** 10\
**Page:** 1

<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 23, 2025, 4:49pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/1 "2025-08-23T16:49:53Z")

</div>

Hello!

I’d like to ask for some help. I want to generate a mesh using `gmsh` for `Calculix`, specifically using only **C3D10** elements.

I have a `cube.geo` file as follows:

```bash
SetFactory("OpenCASCADE");

// 1x1x1 cube
Merge "cube.step";

// Set mesh sizes
Mesh.CharacteristicLengthMin = 0.5; // Minimum size
Mesh.CharacteristicLengthMax = 0.8; // Maximum size

// ?? Generate C3D10 (second order tetrahedron)
Mesh.ElementType = 5; // Tetrahedron
Mesh.ElementOrder = 2;
Mesh.SecondOrderLinear = 0;

// Generate 3D mesh
Mesh 3;

Save "cube.msh";

```

When I open this file in `gmsh` and view the statistics, I get:

- 231 nodes
- 8 points
- 24 lines
- 84 triangles
- 100 tetrahedra

I’m not sure if points, lines and triangles should be there…  
How can I use the generated _.msh_ file in Calculix?

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<div class="post-metadata">

**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\
**Post date:** [August 23, 2025, 6:28pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/2 "2025-08-23T18:28:52Z")

</div>

> [@gmb](#):
>
> I’m not sure if points, lines and triangles should be there…

From Gmsh documentation:

> How can I only save tetrahedral elements (not triangles and lines)?
> 
> By default, if physical groups are defined, the output mesh only contains those elements that belong to physical entities. So to save only 3D elements, simply define one (or more) physical volume(s) and don’t define any physical surfaces, physical curves or physical points.

So just create a physical volume like for example:

`Physical Volume(13) = {1};`

> [@gmb](#):
>
> How can I use the generated _.msh_ file in Calculix?

Better export to .inp since that’s the format CalculiX uses.

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<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 23, 2025, 6:50pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/3 "2025-08-23T18:50:31Z")

</div>

Thank you! It is easier to handle this way!

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<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 23, 2025, 6:54pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/4 "2025-08-23T18:54:45Z")

</div>

Just for completeness, here is the working `.geo` file:

```bash
SetFactory("OpenCASCADE");

Merge "cube.step";

Physical Volume(13) = {1};

// Set mesh sizes
Mesh.CharacteristicLengthMin = 0.5; // Minimum size
Mesh.CharacteristicLengthMax = 0.8; // Maximum size

// Generate C3D10 (second order tetrahedron)
Mesh.ElementType = 5; // Tetrahedra
Mesh.ElementOrder = 2;
Mesh.SecondOrderLinear = 0;

// Generate 3D mesh
Mesh 3;

Save "cube.inp";

```

---

<div class="post-metadata">

**Author:** ![rsmith](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/rsmith/32/1684_2.png) [@rsmith](https://calculix.discourse.group/u/rsmith)\
**Post date:** [August 24, 2025, 11:00am UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/5 "2025-08-24T11:00:04Z")

</div>

I propose a slightly different solution.

Define `Physical Surface` for applying constraints.

Add the following line to your `.geo` file:

```auto
Mesh.SaveGroupsOfElements = -1000;

```

Then `gmsh` will _only_ save volume elements. Any `Physical Surface` you have defined will then consist of nodes from the volume elements. This is _much_ more useful then the default surface sets.

If you want to use `*RIGID BODY` constraints, then it is a good idea to reserve some node numbers for `REF NODE` and `ROT NODE`:

```auto
Mesh.FirstNodeTag = 5; // Reserve node numbers 1-4.

```

A `Physical Volume` is required for gmsh to generate `*ELEMENT`. This also results in an extra `*ELSET`, that is why I usually name the physical volume `ignore`. 🙂

Below is my standard `.geo` template for meshing a STEP-file with C3D10 elements:

```auto
SetFactory("OpenCASCADE");
// Convert coordinates to meters
Geometry.OCCTargetUnit = "M";
Merge "import.stp";

// Describe all physical groups to be saved.
Physical Surface("Nload") = {7};
Physical Surface("Nfix") = {9};
Physical Volume("ignore", 1) = {1};

Mesh.Algorithm = 2; // Automatic
Mesh.ElementOrder = 2; // Create second order elements.
Mesh.SecondOrderIncomplete = 1; // Use 10-node tet elements.

Mesh.MeshSizeFromCurvature = 40;
Mesh.MeshSizeMax = 0.01;

Mesh.FirstNodeTag = 5; // Keep node 1-4 free for ref nodes.
Mesh.Format = 39; // Save mesh as INP format.
Mesh.SaveGroupsOfNodes = 1;
Mesh.SaveGroupsOfElements = -1000; // Only save volume elements.
Mesh.Optimize = 1;

Mesh 3;
OptimizeMesh "Gmsh";
Coherence Mesh;

Save "gmsh-output.inp";

```

If you want to include the generated input in your own input file, make sure that there is only one `*heading` in both of them.

---

<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 25, 2025, 6:42pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/6 "2025-08-25T18:42:49Z")

</div>

Thank you!

It is _almost_ perfect. The ID of the first element is now 117, not 1 and it ignores `Mesh.FirstElementTag`, but no `*ELSET` is generated (I do not need it).

> If you want to include the generated input in your own input file, make sure that there is only one `*heading` in both of them.

Okay, I’ll keep it in mind.

---

<div class="post-metadata">

**Author:** ![shroud](https://avatars.discourse-cdn.com/v4/letter/s/58956e/32.png) [@shroud](https://calculix.discourse.group/u/shroud)\
**Post date:** [August 26, 2025, 1:27pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/7 "2025-08-26T13:27:29Z")

</div>

I have faced a similar problem the last weeks. For me, the .inp export did not work in my pipeline without writing code that opens the exported file and delete unnecessary lines.

To have more flexibility later on, I directly picked the mesh information from the gmsh kernel in python. Physical groups can then be exported with

```python
physical_groups = gmsh.model.getPhysicalGroups()

for dim, tag in physical_groups:
   name = gmsh.model.getPhysicalName(dim, tag)
   entities = gmsh.model.getEntitiesForPhysicalGroup(dim, tag)

   for edim, etag in entities:
       # getNodes
       # getElements

```

Nodes and their coordinates can be extracted by:

```python
(node_tags, node_coords, _) = model.mesh.getNodes(dim=-1, tag=-1, returnParametricCoord=False)
# Then, write the nodes into your data structure.

```

Same goes for the element connectivity.  
You just have to make sure that you do not mix around the order of the lists you will receive. And note that for C3D10 tets, the last two node ids in the connectivity seem to be switched in the gmsh kernel.

This might be a little more effort, but you will have more control on what to export into your calculix mesh, as well as full control over physical entities.

---

<div class="post-metadata">

**Author:** ![rsmith](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/rsmith/32/1684_2.png) [@rsmith](https://calculix.discourse.group/u/rsmith)\
**Post date:** [August 26, 2025, 3:05pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/8 "2025-08-26T15:05:00Z")

</div>

> [@gmb](#):
>
> The ID of the first element is now 117,

That is because the previous element ID’s are taken up by the surface meshes.

I have not found a way to delete _only_ the surface meshes using the gmsh scripting language.

It is however not that difficult to use e.g. Python to post-process the `.inp` file generated by `gmsh` and renumber the elements.

---

<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 26, 2025, 8:25pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/9 "2025-08-26T20:25:15Z")

</div>

Thank you! I’ll look into that. I’m planning to use simple `.geo` files for now, but nice to have a plan B.

---

<div class="post-metadata">

**Author:** ![gmb](https://avatars.discourse-cdn.com/v4/letter/g/edb3f5/32.png) [@gmb](https://calculix.discourse.group/u/gmb)\
**Post date:** [August 26, 2025, 8:27pm UTC](https://calculix.discourse.group/t/gmsh-and-only-c3d10-elements/2859/10 "2025-08-26T20:27:08Z")

</div>

Not really a problem, just bothered me a bit. Thank you very much for your help!