rapidfem 0.13.1

Frequency-domain electromagnetic FEM solver in Rust, with a Python API and an optional local web UI.

RapidFEM

Electromagnetic FEM solver written in Rust, distributed as a Python package on PyPI. Two backends behind one geometry/material/physics API: a frequency-domain solver (Nédélec first-kind order-2 edge elements, complex-symmetric sparse linear algebra) and a time-domain DGTD solver (discontinuous Galerkin, Krylov/ETD exponential time integration, model-order reduction). The solver is scale-invariant, so sub-micron RFIC passives (with GDS / PDK-stack import) solve as reliably as metre-scale structures. Optional Flask-based local UI with code editor and live viewer.

Install

pip install rapidfem            # solver only
pip install rapidfem[ui]        # solver + local UI

Wheels for Windows, Linux, and macOS are built via CI. The Rust core is compiled ahead of time — no Rust toolchain required on the user's machine. Gmsh (Python wheel gmsh) is pulled in automatically and provides the OpenCASCADE-based geometry + mesher used by rapidfem.Geometry.

Quick start (Python API)

import numpy as np
import rapidfem as rf

# Build geometry; attach materials + physics directly to entities
g = rf.Geometry(maxh=rf.lambda_maxh(f_max=12e9))
air = g.box(22.86e-3, 10.16e-3, 30e-3, position=(-11.43e-3, -5.08e-3, 0),
            material=rf.Air())

rf.RectWaveguidePort(air.faces.min(axis="z"))
rf.RectWaveguidePort(air.faces.max(axis="z"))
rf.PEC(*air.faces.unassigned)

g.mesh()

# Define the problem once, run any number of analyses on it
prob = rf.Problem(g)                      # Problem is the frequency-domain ProblemFD
result = prob.sweep(np.linspace(8e9, 12e9, 21))
print(result.frequencies.shape, result.sparams.shape)

# Same Problem can also drive an eigenmode solve or a far-field pattern:
# modes   = prob.eigenmode(target_frequency=10e9, n_modes=6)
# pattern = prob.farfield(result, freq_idx=10, port_idx=0)

See python_src/rapidfem/examples/ for end-to-end runs: microstrip and coupled lines, iris / stepped-impedance filters, patch / Vivaldi / inverted-F antennas (PML + far-field), pyramidal horns, dielectric resonators, and the fd_rfic_* on-chip passives. RFIC geometry comes from a process stack and layout via rapidfem.rfic (rfic.Stack, Geometry.from_gds).

Local UI

rapidfem serve ./my_project/

Opens a browser window with a CodeMirror Python editor, a 3D geometry / mesh / field viewer (raw WebGL2), and S-parameter plots. The geometry view updates on save (Ctrl+S); mesh and solver runs are explicit. Results stream in as the solve runs; fields are fetched on demand as you scrub frequency and port. Use rapidfem.show(g) to send a geometry to the viewer.

Features

• Geometry builder — OpenCASCADE primitives with boolean ops, transforms and fillet/chamfer; ready-made RF structures in rf.structures (coax, microstrip, CPW, stripline, waveguides, helix) build geometry + ports in one call
• RFIC / GDS — rapidfem.rfic process stacks and Geometry.from_gds build on-chip passives, solved scale-invariantly down to sub-micron features
• Canonical Nédélec R2 elements — first-kind order-2 curl–curl vector element, 20 DOFs per tetrahedron
• Excitations — rectangular waveguide ports (arbitrary TE modes), lumped ports (TEM, multi-line voltage integral), coax and wave ports, Floquet plane-wave port (normal incidence), first-order absorbing boundary
• PML — anisotropic stretched-coordinate perfectly matched layer
• Lossy materials — complex permittivity with loss tangent + conductivity, surface impedance for metals, Debye dispersion; cached across sweeps
• Sparse solvers — pure-Rust faer LU baseline; optional MKL PARDISO (complex-symmetric LDLᵀ) on Windows / Linux; Apple Accelerate Bunch-Kaufman on macOS (~3× faster than faer)
• Frequency sweep — assembles E/B once, refactors only the frequency- dependent K per point, reuses the symbolic LU pattern
• Eigenmode solver — shift-invert Lanczos on the complex-symmetric system
• Adaptive refinement — residual error estimator with Dörfler marking, exports a size field for gmsh re-meshing
• Output — Touchstone (.s1p/.s2p/.snp), VTK field export, far-field NFFT
• Parallel assembly — rayon-based element matrix evaluation

Time-domain backend (DGTD)

ProblemTD, behind the same API, compiles a structure into an explicit linear ODE dy/dt = A·y and exposes it as a model at every level:

• DGTD — nodal discontinuous Galerkin on tetrahedra, upwind or energy-conserving central flux
• Exponential time integration — matrix-free Krylov/ETD propagator, exact for the linear system at any step size (no CFL limit)
• Model export / reduction — the RHS, the verbatim sparse operator A, an exponential stepper, or Krylov-projected reduced models
• Materials — heterogeneous, lossy, anisotropic and Debye dispersive media; matched absorbing layers; periodic boundaries
• Output — field probes, RFT transfer function, VTK field-animation export

import rapidfem as rf

ptd  = rf.ProblemTD.box(size=(1, 1, 1), cells=(2, 2, 2), order=2)
traj = ptd.transient(y0, dt=0.02, steps=200)   # turnkey transient
rom  = ptd.reduce(y0, dim=60)                   # model-order reduction
A    = ptd.state_space()                        # the verbatim operator

Method notes and the ProblemTD API are in docs/td-backend.md.

Solver backends

Solver	Type	Notes
faer	General sparse LU	Pure Rust, no native dependencies — always available
MKL PARDISO	Complex-symmetric LDLᵀ	Fastest on Windows / Linux; opt-in, needs mkl_rt on PATH
Apple Accelerate	Sparse Bunch-Kaufman LDLᵀ	macOS only; ~3× faster than faer, ships with macOS

Select with RAPIDFEM_SOLVER ("auto", "pardiso", "accelerate", "faer"), set before import rapidfem. Default "auto" tries PARDISO → Accelerate → faer. Optional MKL: conda install mkl / pip install mkl (ensure mkl_rt is on PATH).

Performance

WR-90 iris waveguide driven sweep, 10 GHz, 2-port:

Mesh	DOFs	PARDISO	faer
693 tets	5 512	0.14 s	0.22 s
1 096 tets	8 382	0.06 s	0.45 s
2 595 tets	19 196	0.17 s	1.39 s
3 284 tets	23 968	0.21 s	1.98 s

Larger: 327 k DOFs driven sweep (PARDISO) ~5 s/freq; 905 k DOFs eigenmode (3-turn spiral, shift-invert Lanczos) ~54 s.

Verification

cargo test --release checks element-level functions to machine precision (1e-12 – 1e-16). End-to-end S-parameter accuracy is tracked in tests/validation/ against analytical solutions and reference solvers.

License

GPL-3.0-or-later with the Gmsh additional permission — see LICENSE. Copyright (C) Milan Rother and rapidfem contributors; commercial terms available.