atom-hifi 0.5.1

Atom-HiFi: atomistic high-fidelity representative-set selection framework

Atom-HiFi

Atomistic High-Fidelity representative-set selection framework.

Applications include:

• MLIP training-set curation and active-learning loops
• Chemical motif identification and distribution analysis
• Diversity-aware structure sampling from large databases

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What is Atom-HiFi?

Atom-HiFi finds the smallest subset S of a structure library that achieves high Fidelity — meaning S covers the library's atomic-environment diversity efficiently, without redundancy. Agnostic to the downstream task.

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Key concepts

Fidelity = L / R

Fidelity is the single optimisation objective. Like a HiFi audio system, it has two channels — L (Left) and R (Right) — whose ratio is maximised. High Fidelity means the selection is both faithful to the library distribution (high Likeness) and compact (low Redundancy).

L — Likeness measures how faithfully S reproduces the library's atomic-environment distribution. Each atom is assigned to a microstate (Voronoi cell in whitened descriptor space from k-means); L is the Shannon entropy ratio over those populations:

L = H(sub) / H(lib)      H = -Σ p_i ln p_i

Shannon entropy H measures distributional diversity — how evenly the population is spread across microstates. L = 1: S perfectly reproduces the library's diversity. L < 1: some environments are under-represented; e.g. L = 0.95 means S retains 95% of the library's distributional diversity.

R — Redundancy measures how many atoms are packed per occupied microstate, relative to the full library:

R = (N_sub / k_occ^sub) / (N_lib / k_occ^lib)

R = 1: same atoms-per-microstate density as the full library (no compression). R < 1: redundancy has been removed; e.g. R = 0.4 means 60% of redundant atoms are eliminated while the occupied microstate coverage is preserved.

The scan sweeps a bandwidth c (scaling factor on ε_noise) and finds c* that maximises Fidelity subject to L ≥ L_TOL (default 0.90). The optimal c* sits at the elbow of the L/R curve — the point where further reducing redundancy begins to cost meaningful distributional diversity.

ED-SOAP descriptor

Embedded Double SOAP — two concatenated SOAP power-spectrum vectors per atom: one short-range (bonding geometry) and one long-range (coordination shell), normalised by a system-specific lengthscale. No GPU required. The full parameter set is exposed in hifi_workflow_tutorial.py under the EDS_* variables.

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Installation

Step 1 — install decaf (Descriptor Embedding and Clustering for Atomistic-environment Framework — the clustering backend; not on PyPI):

pip install git+https://gitlab.mpcdf.mpg.de/klai/decaf.git

Step 2 — install Atom-HiFi:

pip install atom-hifi

Python ≥ 3.9 required.

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Quick start

pip install atom-hifi installs the atom-hifi command. Write a starter config, edit it, and run:

atom-hifi init                 # writes a commented config.yaml
# edit config.yaml (at minimum: paths.lib_path, paths.focus_elements)
atom-hifi run config.yaml 2>&1 | tee run.out

The generated config.yaml documents every setting inline. The minimum to edit:

paths:
  lib_path: train_structs.xyz   # ASE-readable structure library
  focus_elements: [Ni, O]       # elements to cluster on
  output_dir: fr_results
descriptor:
  kind: eds                     # 'eds' or 'ace'

Python API / custom descriptors

The CLI supports the eds and ace descriptors. A custom descriptor is a Python callable and is supplied via the Python API. hifi_workflow_tutorial.py is the annotated example (included in the repo; pip-only users can fetch it):

curl -O https://gitlab.mpcdf.mpg.de/yhsong/atom-hifi/-/raw/main/hifi_workflow_tutorial.py

Edit its top-level variables (including DESCRIPTOR_FN) and run python hifi_workflow_tutorial.py, or call the runner directly:

from atom_hifi.runner import run
run({'paths': {'lib_path': 'train_structs.xyz', 'focus_elements': ['Ni', 'O']},
     'descriptor': {'kind': 'custom', 'custom_fn': my_descriptor_fn}})

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Output files

File	Description
representatives.xyz	Selected representative structures
fine_scan.out	L, R, F (=L/R), |S|, atoms for every fine-scan point
hifi_final.png	Coarse + fine Fidelity (F = L/R) scan diagnostic plot
learning_curve.png	AL loop convergence (only with RUN_LOOP=True)
eps_noise_raw.npz	Cached per-element ε_noise values
desc_lib.pkl	Cached per-structure descriptors
surroundings_{el}.xyz	Per-group coordination spheres (EXTRACT_SURROUNDINGS=True)

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Configuration reference

All settings live in config.yaml (run atom-hifi init to generate a fully commented template). Keys are grouped:

Group	Keys
paths	lib_path, patient_path, focus_elements, output_dir
descriptor	kind, eds.{lengthscale, s_cut, s_nmax, s_lmax, l_cut, l_nmax, l_lmax, periodic, r_cut}, ace.{model_path, device, r_cut}
selection	method (mu_tiebreak recommended)
scan	l_tol, n_coarse, n_fine, n_jobs, c_factor_range
eps_noise	per_species, temperature (K; sets σ_thermal ∝ √T/√mass for ε_noise calibration)
loop / grid / nsga2	run + per-stage tuning
refit	delta, grid_point
output	delta_pick, extract_surroundings

Unknown keys are rejected. The same configuration can be passed as a nested dict to atom_hifi.runner.run(...); hifi_workflow_tutorial.py is the annotated Python-API equivalent.

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Advanced usage

Active-learning loop (RUN_LOOP=True)

Iteratively expands the training pool by sampling batches from the full library. Inner iterations use a coarse scan only; one final fine scan runs at the end. Set INITIAL_SAMPLE and LOOP_SKIP_FINE_SCAN to control the initial pool size and inner-scan resolution.

Per-element ND grid scan (RUN_GRID_SCAN=True)

Sweeps independent c-factors per focus element on a Cartesian grid, reusing cached per-element DECAF fits from the 1-D scan. Cost is O(n^N_el) cover evaluations instead of O(n^N_el × N_el) DECAF fits — tractable for N_el ≤ 3–4. Results in scan_grid.csv and scan_grid_report.png.

NSGA-II Pareto optimisation (RUN_NSGA2=True)

Stochastic multi-objective optimisation of per-element c-factors via NSGA-II (requires pymoo). Use when the grid is too large (N_el ≥ 4) or you want a continuous Pareto front. Results in pareto_front.csv and three diagnostic PNGs.

Representative environment extraction (EXTRACT_SURROUNDINGS=True)

Exports the local coordination sphere around the centroid-closest atom of each DECAF group. Two modes: 'sphere' (non-periodic ASE Atoms cluster) and 'full_structure' (original cell with center/neighbour/rest tags). Output: surroundings_{el}.xyz per focus element.

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Citation

If you use Atom-HiFi in your research, please cite:

[paper in preparation — citation will be added upon publication]