The same msh model is used by dividing it into two regions.

[CherryJGit]

I have a tetrahedral mesh of a sphere in an msh model. I hope that the tetrahedral elements in the upper half of the sphere cannot be eliminated, while those in the lower half can be. Moreover, there should be a regional division between the upper half and the lower half, rather than a determination based on a standard. For example, the tetrahedral elements in the region enclosed by the BoxROI should not be eliminated. I'm not sure if this is feasible. I tried to write an scn file with the following content. However, no matter how I modify it, errors will always occur. I will put the content of the file below. Maybe my train of thought is wrong.And after it runs, due to the influence of gravity, it will split into two parts on its own. What I hope for is the whole. Even so, I spent a long time consulting various AIs, such as Grok, GPT, DeepSeek, etc., before determining this method. I hope everyone can help me check where the problem lies, or provide an scn file for other possible approaches. I would be extremely grateful. Thank you in advance.

<CollisionPipeline />
<DefaultVisualManagerLoop />
<DefaultAnimationLoop />
<BruteForceBroadPhase/>
<BVHNarrowPhase/>
<CollisionResponse response="PenalityContactForceField" />
<MinProximityIntersection name="Proximity" alarmDistance="0.8" contactDistance="0.5" />

<Node name="ProtectedLiver">
    <MechanicalObject name="Volume" position="@../Subset.pointsInROI" />
    <TetrahedronSetTopologyContainer name="TetraContainer" tetrahedra="@../Subset.tetrahedraInROI" />
    <TetrahedronSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" drawTetrahedra="0" drawScaleTetrahedra="0.7"/>
    <DiagonalMass massDensity="4.0" />
    <TetrahedronFEMForceField name="FEM" youngModulus="3000" poissonRatio="0.3" method="large" computeVonMisesStress="1"/>
    <EulerImplicitSolver name="cg_odesolver" printLog="false" rayleighStiffness="0.01" rayleighMass="1.0" />
    <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
    <PlaneForceField normal="0 1 0" d="-5" stiffness="100000" showPlane="1" showPlaneSize="20"/>
    <BoxROI name="FixedROI" box="-1 6 -0.5 1 7 1.5" position="@Volume.rest_position" drawBoxes="1"/>
    <FixedProjectiveConstraint indices="@FixedROI.indices" />
    
    <Node name="LiverSurface">
        <TriangleSetTopologyContainer name="TriContainer" fileTopology="" tags=" " />
        <TriangleSetTopologyModifier name="Modifier" />
        <TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" />
        <Tetra2TriangleTopologicalMapping input="@../TetraContainer" output="@TriContainer" />
        <TriangleCollisionModel />
        <Node name="LiverVisu">
            <OglModel name="Visual" color="red" />
            <IdentityMapping input="@../../Volume" />
        </Node>
    </Node>
</Node>

<Node name="ErasableLiver">
    <MechanicalObject name="Volume" position="@../Subset.pointsOutROI" />
    <TetrahedronSetTopologyContainer name="TetraContainer" tetrahedra="@../Subset.tetrahedraOutROI" />
    <TetrahedronSetTopologyModifier name="Modifier" />
    <TetrahedronSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" drawTetrahedra="0" drawScaleTetrahedra="0.7"/>
    <DiagonalMass massDensity="4.0" />
    <TetrahedronFEMForceField name="FEM" youngModulus="3000" poissonRatio="0.3" method="large" computeVonMisesStress="1"/>
    <EulerImplicitSolver name="cg_odesolver" printLog="false" rayleighStiffness="0.01" rayleighMass="1.0" />
    <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
    <PlaneForceField normal="0 1 0" d="-5" stiffness="100000" showPlane="1" showPlaneSize="20"/>
    
    <Node name="LiverSurface">
        <TriangleSetTopologyContainer name="TriContainer" fileTopology="" tags=" " />
        <TriangleSetTopologyModifier name="Modifier" />
        <TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" />
        <Tetra2TriangleTopologicalMapping input="@../TetraContainer" output="@TriContainer" />
        <TriangleCollisionModel />
        <Node name="LiverVisu">
            <OglModel name="Visual" color="red" />
            <IdentityMapping input="@../../Volume" />
        </Node>
    </Node>
</Node>

The error log brought by this code is：
########## SIG 11 - SIGSEGV: segfault ##########
sofa::helper::BackTrace::dump
sofa::helper::BackTrace::sig
seh_filter_exe
seh_filter_exe
_C_specific_handler
_chkstk
RtlRaiseException
KiUserExceptionDispatcher
sofa::component::solidmechanics::fem::elastic::TetrahedralCorotationalFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::computeRotationLarge
sofa::component::solidmechanics::fem::elastic::TetrahedronFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::computeVonMisesStress
sofa::component::solidmechanics::fem::elastic::TetrahedronFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::handleEvent
sofa::simulation::PropagateEventVisitor::processObject
sofa::simulation::PrintVisitor::treeTraversal
sofa::simulation::PrintVisitor::treeTraversal
sofa::simulation::PropagateEventVisitor::processNodeTopDown
sofa::simulation::graph::DAGNode::executeVisitorTopDown
sofa::simulation::graph::DAGNode::executeVisitorTopDown
sofa::simulation::graph::DAGNode::doExecuteVisitor
sofa::simulation::Node::executeVisitor
sofa::simulation::DefaultAnimationLoop::propagateAnimateEndEvent
sofa::simulation::DefaultAnimationLoop::step
sofa::simulation::node::animate
sofa::gui::qt::RealGUI::step
QMetaObject::activate
QTimer::timerEvent
QObject::event
QApplicationPrivate::notify_helper
QApplication::notify
QCoreApplication::notifyInternal2
QEventDispatcherWin32::event
QApplicationPrivate::notify_helper
QApplication::notify
QCoreApplication::notifyInternal2
QCoreApplicationPrivate::sendPostedEvents
qt_plugin_query_metadata
QEventDispatcherWin32::processEvents
CallWindowProcW
DispatchMessageW
QEventDispatcherWin32::processEvents
qt_plugin_query_metadata
QEventLoop::exec
QCoreApplication::exec
sofa::gui::qt::RealGUI::mainLoop
sofa::gui::common::GUIManager::MainLoop
sofa::gui::common::GUIManager::MainLoop
sofa::gui::common::GUIManager::MainLoop
BaseThreadInitThunk
RtlUserThreadStart
[WARNING] [Sofa.Simulation.Graph] the library has not been cleaned up (sofa::simulation::graph::cleanup() has never been called, see sofa/helper/init.h)
[WARNING] [SofaSimulationCommon] the library has not been cleaned up (sofa::simulation::common::cleanup() has never been called, see sofa/helper/init.h)

[epernod]

Hi,

sorry I don't understand what you are trying to simulate. Could you explain again, maybe with a drawing or illustration.
You have 1 mesh, you want 2 regions? and then you do what?

Best regards