XPCSJAX - JAX-native NLSQ fitting for XPCS.

Disclaimer: XPCSJAX is pre-release software under active development and testing. APIs, model implementations, and numerical results may change without notice. Outputs have not been independently validated for all use cases. Use at your own risk and verify critical results against established reference data before relying on them in research or industrial applications.

XPCSJAX consolidates the homodyne and heterodyne analysis pipelines — both now deprecated in its favor — into one package with a shared engine and config-driven physics-model dispatch. It implements the transport-coefficient framework of He et al. PNAS 2024 and He et al. PNAS 2025 for characterizing nonequilibrium dynamics in flowing soft-matter systems.

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Install

To get every feature and avoid missing-dependency issues, install with the all extra (pulls every optional dependency — GUI, fast-viz, plus the dev/docs/packaging tooling).

With uv (recommended):

uv pip install "xpcsjax[all]"

In a mamba/conda environment, install directly from the imewei Anaconda.org channel:

mamba install imewei::xpcsjax
# or: conda install imewei::xpcsjax

Or use pip inside the same environment — or any other virtualenv (venv, virtualenv, pyenv):

pip install "xpcsjax[all]"

For a minimal core install (NLSQ fitting only, no GUI), drop the extra: uv pip install xpcsjax or pip install xpcsjax — this already includes datashader-accelerated plotting. The xpcsjax[gui] extra adds the desktop workbench.

Python 3.12+ required, CPU-only in v0.1 (GPU support is v0.2+). Runtime dependencies are managed via pyproject.toml and mirror what the source homodyne package pins (versions of jax, nlsq, evosax, h5py, jaxopt, psutil, tqdm, etc.).

From source (development): clone the repo and use an editable install — uv sync && uv pip install -e ".[dev]". See docs/source/development/contributing.rst.

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Quickstart

from xpcsjax import load_xpcs_data, fit_nlsq

data   = load_xpcs_data("config.yaml")
result = fit_nlsq(data, "config.yaml")
print(result.parameters)

The YAML config's analysis_mode field selects the physics model and parameter set:

analysis_mode	Lineage	Model	Physics params
static_isotropic	homodyne	Equilibrium diffusion, angle-collapsed	3
static_anisotropic	homodyne	Same physics; angle-resolved data prep	3
laminar_flow	homodyne	Diffusion + sinc-shear	7
two_component (or heterodyne)	heterodyne	Two-component: reference + sample + velocity + mixing	14

fit_nlsq returns a single OptimizationResult for every mode (result.parameters is a NumPy array). Heterodyne (two_component) fits all phi angles jointly and packs the multi-angle result into that same object. Two per-angle scaling parameters (contrast, offset) are appended automatically for every azimuthal angle in all modes.

Data flow

YAML config --> XPCSDataLoader(HDF5) --> HomodyneModel / HeterodyneModel --> NLSQ engine --> Results (JSON + NPZ)

save_results (xpcsjax/service/persist.py) writes nlsq_result.json (fitted parameters, uncertainties, χ², diagnostics) and nlsq_result.npz (correlation and residual arrays) to the configured output directory.

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Running XPCSJAX

Three front-ends drive the same YAML config and NLSQ engine — pick whichever fits your workflow.

Command line. The xpcsjax console script (alias xj) runs a flag-driven fit:

xpcsjax --config analysis.yaml                          # run an NLSQ fit
xpcsjax --config analysis.yaml --output ./results       # override the output dir
xpcsjax --config analysis.yaml --multistart --multistart-n 16
xpcsjax --config analysis.yaml --plot-experimental-data # plot only, skip the fit

Exit codes: 0 converged · 2 ran but did not converge (outputs still written) · 1 error · 130 interrupted. xjexp / xjsim are experimental-/simulated-data plotting shortcuts. See the CLI guide for the full command reference.

Interactive workbench (GUI). Needs the gui extra (pip install "xpcsjax[gui]"). Launch the PySide6 analysis workbench with:

xpcsjax-gui      # or: xj-gui

Load a config, run fits, and browse per-angle results and residual maps. The fit runs in a JAX-free worker process so the UI stays responsive; xpcsjax-gui -platform offscreen does a headless smoke run.

Shell completion. After install, wire up tab-completion (bash/zsh; fish is a non-fatal no-op) and optional XLA flags into your environment:

xpcsjax-post-install   # interactive: completion + XLA_FLAGS into the venv activate script
xpcsjax-cleanup        # remove what post-install added

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Physics models

XPCSJAX fits two-time intensity correlation functions $c_2(\vec{q}, t_1, t_2)$. All time integrals are evaluated numerically via cumulative trapezoid on the discrete time grid — no analytical antiderivatives — so the general power-law forms stay correct.

Homodyne (static_*, laminar_flow)

Single-component scattering where correlation decay encodes diffusion and shear. The laminar-flow two-time kernel (xpcsjax core/physics_nlsq.py; derived in docs/source/theory/homodyne_model.rst) is

$$c_2(\vec{q}, t_1, t_2) = c_{\text{offset}}(\phi) + \beta(\phi) \exp\left(-q^2\int_{t_1}^{t_2} J(t') dt'\right) \mathrm{sinc}^2\left(\frac{q h \cos(\phi - \phi_0) \Gamma(t_1, t_2)}{2\pi}\right)$$

with $\mathrm{sinc}(x) = \sin(\pi x)/(\pi x)$, accumulated strain $\Gamma(t_1, t_2) = \int_{t_1}^{t_2}\dot{\gamma}(t) dt$, rheometer gap $h$ (instrument geometry, not fitted), flow angle $\phi_0$, and per-angle scaling $\beta(\phi)$ / $c_{\text{offset}}(\phi)$ (the contrast / offset parameters). The static modes drop the shear term ($\mathrm{sinc}^2 \to 1$). Transport and shear follow power-law forms:

$$J(t) = D_0 t^{\alpha} + D_{\text{offset}} \qquad \dot{\gamma}(t) = \dot{\gamma}0 t^{\beta} + \dot{\gamma}{\text{offset}}$$

Registry parameter names (xpcsjax/config/parameter_registry.py) and their defaults:

Group	Parameter	Description	Default	Units
Diffusion	D0	Diffusion prefactor	1e3	Ų/s
	alpha	Transport exponent (0 = Wiener, 1 = ballistic)	0.5	—
	D_offset	Transport rate offset	10	Ų/s
Shear (laminar_flow only)	gamma_dot_t0	Shear-rate prefactor	0.01	s⁻¹
	beta	Shear-rate exponent (0 = constant shear)	0.5	—
	gamma_dot_t_offset	Shear-rate offset	0.0	s⁻¹
Flow angle	phi0	Flow angle offset relative to q-vector	0.0	degrees
Per-angle scaling	contrast	Optical (speckle) contrast	0.5	—
	offset	Baseline offset	1.0	—

The static modes use the 3 diffusion parameters; laminar_flow adds the 3 shear parameters and the flow angle (7 physics parameters total).

Heterodyne (two_component)

Two-component scattering (PNAS 2025 SI Eqs. S-77–S-98): light from a moving sample interferes with a static reference, and the cross-term oscillates at a frequency set by the sample velocity. The two-time correlation (Eq. S-95) is

$$c_2(\vec{q}, t_1, t_2) = 1 + \frac{\beta}{f^2}\left[C_{\text{ref}} + C_{\text{sample}} + C_{\text{cross}}\right]$$

$$C_{\text{ref}} = [x_r(t_1)x_r(t_2)]^2 \exp\left(-q^2\int_{t_1}^{t_2} J_r dt'\right) \qquad C_{\text{sample}} = [x_s(t_1)x_s(t_2)]^2 \exp\left(-q^2\int_{t_1}^{t_2} J_s dt'\right)$$

$$C_{\text{cross}} = 2 x_r(t_1)x_r(t_2)x_s(t_1)x_s(t_2) \exp\left(-\tfrac{1}{2}q^2\int_{t_1}^{t_2}[J_s + J_r] dt'\right)\cos\left[q\cos(\varphi)\int_{t_1}^{t_2}\mathbb{E}[v] dt'\right]$$

where $x_s(t)$ is the sample fraction, $x_r = 1 - x_s$ the reference fraction, $\varphi$ the angle between velocity and $\vec{q}$, and $f^2 = [x_s(t_1)^2 + x_r(t_1)^2][x_s(t_2)^2 + x_r(t_2)^2]$ normalizes so that $c_2(t, t) = 1 + \beta$ on the diagonal. The fit wraps the correlation with per-angle scaling, $c_2^{\text{model}} = \text{offset} + \text{contrast}\times(C_{\text{ref}} + C_{\text{sample}} + C_{\text{cross}})/f^2$.

Each transport coefficient and the velocity follow power laws, and the sample fraction is time-dependent — 14 physics parameters in five groups:

Registry parameter names (xpcsjax/config/parameter_registry.py, canonical order) and their defaults:

Group	Parameters	Rate function	Defaults	Units
Reference transport (3)	D0_ref, alpha_ref, D_offset_ref	$J_r(t) = D_{0,r} t^{\alpha_r} + D_{\text{offset},r}$	1e4, 0, 0	Ų/s, —, Ų/s
Sample transport (3)	D0_sample, alpha_sample, D_offset_sample	$J_s(t) = D_{0,s} t^{\alpha_s} + D_{\text{offset},s}$	1e4, 0, 0	Ų/s, —, Ų/s
Velocity (3)	v0, v_beta, v_offset	$v(t) = v_0 t^{\beta} + v_{\text{offset}}$	1e3, 1, 0	Å/s, —, Å/s
Sample fraction (4)	f0, f1, f2, f3	$f_s(t) = f_0 \exp\big(f_1(t - f_2)\big) + f_3$	0.5, 0, 0, 0	—, —, —, —
Flow angle (1)	phi0_het	—	0	degrees

The velocity exponent is v_beta and the flow angle phi0_het in the registry — deliberately distinct from homodyne's beta / phi0 to avoid name collisions. As with homodyne, 2 per-angle scaling parameters (contrast, offset) are tracked per azimuthal angle but live outside the 14-element physics vector. Per-angle scaling modes constant / averaged / individual (resolved from auto) reach full parity with the source heterodyne package's fit_nlsq_multi_phi.

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Citation

XPCSJAX implements the transport-coefficient framework introduced in:

@article{He2024,
  author  = {He, Hongrui and Liang, Heyi and Chu, Miaoqi and Jiang, Zhang and
             de Pablo, Juan J and Tirrell, Matthew V and Narayanan, Suresh
             and Chen, Wei},
  title   = {Transport coefficient approach for characterizing nonequilibrium
             dynamics in soft matter},
  journal = {Proceedings of the National Academy of Sciences},
  volume  = {121},
  number  = {31},
  year    = {2024},
  doi     = {10.1073/pnas.2401162121}
}

@article{He2025,
  author  = {He, Hongrui and Liang, Heyi and Chu, Miaoqi and Jiang, Zhang and
             de Pablo, Juan J and Tirrell, Matthew V and Narayanan, Suresh
             and Chen, Wei},
  title   = {Bridging microscopic dynamics and rheology in the yielding
             of charged colloidal suspensions},
  journal = {Proceedings of the National Academy of Sciences},
  volume  = {122},
  number  = {42},
  year    = {2025},
  doi     = {10.1073/pnas.2514216122}
}

To cite the software itself:

@software{XPCSJAX,
  author      = {Chen, Wei},
  title       = {XPCSJAX: JAX-native NLSQ fitting for X-ray Photon Correlation Spectroscopy},
  year        = {2026},
  version     = {0.1.4},
  institution = {Argonne National Laboratory},
  url         = {https://github.com/imewei/xpcsjax}
}

License

MIT. See LICENSE.

Authors

• Wei Chen (weichen@anl.gov) — Argonne National Laboratory