Solver and Equation Compatibility¶

In one sentence: FreeCAD FEM supports four solvers, each covering a different subset of physics — knowing which solver handles which analysis type, and which boundary conditions and equations are required, prevents the most common "solver won't run" errors.

The four solvers¶

Solver	Executable	Physics covered	Best for
CalculiX	`ccx`	Structural, thermal, thermo-mechanical, buckling, frequency, electromagnetic	Most structural and thermal work
Elmer	`ElmerSolver`	Electrostatic, electromagnetic, fluid flow, heat, structural deformation	Multiphysics, EM, flow
Mystran	`mystran`	Linear static structural only	Quick linear checks (NASTRAN-format)
Z88	`z88r`	Linear static structural only	Quick linear checks (alternative)

Each solver requires its executable to be installed separately and the path configured in Edit → Preferences → FEM → [SolverName].

CalculiX — analysis type matrix¶

CalculiX is controlled by a single Analysis Type property. All required setup (mesh, materials, BCs) is the same structure regardless of type — only the result outputs differ.

Analysis Type	Solves	Required BCs	Optional
`static`	Linear/nonlinear deformation and stress	Fixed Constraint + at least one load	Spring, Contact
`frequency`	Natural frequencies and mode shapes (modal)	Fixed Constraint	No loads needed — loads are ignored
`thermomech`	Temperature distribution and thermal stress	Temperature Constraint + Heat Flux or Body Heat Source	Mechanical loads for coupled thermo-mechanical
`buckling`	Critical load multiplier (linear buckling)	Same as static	—
`check`	Mesh quality check only	None required	—
`electromagnetic`	Electrostatic field	Electrostatic Potential	Electric Charge

Frequency analysis ignores all loads

Modal analysis computes natural vibration modes of the structure. Applied forces, pressures, and gravity loads are not used. If you want stress at a specific mode, run a separate static analysis.

Geometrical Nonlinearity must be enabled for large-deformation problems

For problems with displacements greater than ~5% of the part dimensions, enable GeometricalNonlinearity = nonlinear on the CalculiX solver object. The default linear setting uses a small-strain formulation.

Elmer — equation-based physics¶

Elmer has no built-in analysis type. You define the physics by adding equation objects to the Analysis container. Each equation object adds one governing PDE to the solve.

Equation	Physics	Required BCs	Required material properties
Elasticity	Linear structural (small strain)	Displacement	Young's modulus, Poisson ratio, density
Deformation	Nonlinear structural (large strain)	Displacement	Young's modulus, Poisson ratio, density
Heat	Heat conduction/convection	Temperature and/or Heat Flux	Thermal conductivity λ, specific heat c, density ρ
Flow	Navier-Stokes fluid flow	Flow Velocity on inlet/wall	Viscosity, density
Electrostatic	Electric potential ∇·(ε∇V) = −ρ	Electrostatic Potential	Permittivity ε
Static Current	DC conduction ∇·(σ∇V) = 0	Electrostatic Potential on terminals	Conductivity σ
Magnetodynamic	Time-harmonic (AC) EM fields	Magnetization or Current Density	Permeability, conductivity
Magnetodynamic 2D	2-D planar/axisymmetric EM	Same as Magnetodynamic	Same as Magnetodynamic
Electricforce	Electrostatic body force	Requires Electrostatic to be solved first	—
Flux	Heat or electric flux (post-processing)	Requires Heat or Electrostatic to be solved first	—

Elmer multiphysics combinations¶

Elmer's power lies in coupling multiple equations in one solve:

Combination	Use case
Flow + Heat	Conjugate heat transfer (fluid cooling a solid)
Electrostatic + Electricforce + Elasticity	Electromechanical actuator (MEMS)
Magnetodynamic + Heat	Induction heating (Joule heating in conductor)
Elasticity + Heat	Thermal stress (uncoupled)

Add all required equations to the same Analysis. Elmer solves them simultaneously (or sequentially for weakly coupled systems).

Mystran¶

Linear static structural only. Equivalent to CalculiX static with GeometricalNonlinearity = linear. Use when CalculiX is not installed or when a NASTRAN-format workflow is required.

Required: Fixed Constraint + at least one mechanical load.

Z88¶

Linear static structural only. Equivalent to CalculiX static with GeometricalNonlinearity = linear. Alternative lightweight solver.

Required: Fixed Constraint + at least one mechanical load.

Choosing the right solver¶

Scenario	Solver to use
Static stress analysis — linear	CalculiX `static` or Mystran or Z88
Static stress analysis — nonlinear (large deflection, plasticity)	CalculiX `static` + Geometrical Nonlinearity
Modal (natural frequency) analysis	CalculiX `frequency`
Buckling analysis	CalculiX `buckling`
Steady-state or transient heat conduction	CalculiX `thermomech` or Elmer Heat
Thermal stress (heat + structural)	CalculiX `thermomech` (coupled)
Fluid flow (CFD)	Elmer Flow
Conjugate heat transfer (fluid + solid)	Elmer Flow + Heat
Electrostatic field	CalculiX `electromagnetic` or Elmer Electrostatic
DC current flow and Joule heating	Elmer Static Current + Heat
AC electromagnetic / eddy currents	Elmer Magnetodynamic
MEMS / electromechanical coupling	Elmer Electrostatic + Electricforce + Elasticity

Common setup errors¶

Error	Likely cause	Fix
Solver runs but produces zero displacement	No load applied, or loads applied to wrong face	Verify loads are on the loaded face; check magnitudes are non-zero
Solver fails: "No boundary conditions"	Missing required BC for the chosen equation/analysis type	Add at minimum one Dirichlet BC (fixed constraint, temperature)
Solver fails: "No material"	Analysis has no material assigned to the mesh	Add a Material Solid and assign it to the body
CalculiX frequency: zero modes	All DOF are fixed (fully clamped)	Remove excess fixity — leave enough free DOF for vibration
Elmer won't converge (Flow)	Turbulent flow with laminar Navier-Stokes	Reduce Reynolds number or use simpler geometry
CalculiX static: large displacement, wrong results	Nonlinear problem solved linearly	Enable GeometricalNonlinearity = nonlinear

Python API¶

import ObjectsFem
import FreeCAD as App

doc = App.ActiveDocument
analysis = doc.getObject("Analysis")

# Add a CalculiX solver
calc = ObjectsFem.makeSolverCalculiX(doc, "CalculiX")
calc.AnalysisType = "static"
calc.GeometricalNonlinearity = "linear"
analysis.addObject(calc)

# Add an Elmer solver
elmer = ObjectsFem.makeSolverElmer(doc, "Elmer")
analysis.addObject(elmer)

# Add Elmer equations
heat_eq = ObjectsFem.makeEquationHeat(doc, elmer)
flow_eq = ObjectsFem.makeEquationFlow(doc, elmer)

doc.recompute()