# Constrained vertex on fault error

**URL:** <https://community.geodynamics.org/t/constrained-vertex-on-fault-error/700>\
**Category:** PyLith\
**Created:** [July 1, 2019, 10:15pm UTC](https://community.geodynamics.org/t/constrained-vertex-on-fault-error/700 "2019-07-01T22:15:52Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![albarodriguez](https://avatars.discourse-cdn.com/v4/letter/a/f05b48/32.png) [@albarodriguez](https://community.geodynamics.org/u/albarodriguez)\
**Post date:** [July 1, 2019, 10:15pm UTC](https://community.geodynamics.org/t/constrained-vertex-on-fault-error/700/1 "2019-07-01T22:15:52Z")

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Hi, I am getting this error using a modified version of example 06 in hex8. This is the same code I modified at the workshop, I am just implementing it on a new mesh, which seems to be the issue. The error seems clear, I just do not understand what constrained means in this context.

"RuntimeError: Vertex with label ‘69102’ on negative side of fault ‘fault’ is constrained.

Fault vertices cannot be constrained."

Thank you for your help,

Alba

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**Author:** ![baagaard](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/baagaard/32/615_2.png) [@baagaard](https://community.geodynamics.org/u/baagaard)\
**Post date:** [July 1, 2019, 10:31pm UTC](https://community.geodynamics.org/t/constrained-vertex-on-fault-error/700/2 "2019-07-01T22:31:19Z")

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You cannot have an overlap between a Dirichlet boundary condition and the fault. In other words, you have one or more vertices that are in a nodeset for the fault and in a nodeset for a Dirichlet boundary condition. The remedy is to remove the nodeset for the Dirichlet boundary condition. You can use group boolean operations on groups in CUBIT/Trelis for this.

Here is how we create a nodeset `face_zeg_nofault` which has all vertices on the -z boundary that are not on the fault from the group `face_zneg` (all vertices on the -z boundary) and group `fault` (all vertices on the fault):

```auto
group "face_zneg_nofault" add node in face_zneg
group "face_zneg_nofault" remove node in fault
nodeset 16 group face_zneg_nofault
nodeset 16 name "face_zneg_nofault"

```

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<div class="post-metadata">

**Author:** ![albarodriguez](https://avatars.discourse-cdn.com/v4/letter/a/f05b48/32.png) [@albarodriguez](https://community.geodynamics.org/u/albarodriguez)\
**Post date:** [July 3, 2019, 8:24pm UTC](https://community.geodynamics.org/t/constrained-vertex-on-fault-error/700/3 "2019-07-03T20:24:46Z")

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thank you – that did it.  
Script is running now and producing desirable results.

Alba