Module experimental_status 

E4 – Experimental Module Validation & Retention Criteria

This module documents which experimental methods remain gated behind feature flags and the criteria required for eventual promotion to core.

Promoted to Core (E3)

Module	Core path	Justification
EEQ charges	charges_eeq	Topology-free partial charges, O(N²), validated vs Gasteiger
D4 dispersion	dispersion	BJ-damped DFT-D4 energy, complements UFF/MMFF94
ALPB solvation	solvation_alpb	Klamt-corrected solvation, improves over plain GB
sTDA UV-Vis	spectroscopy::stda_uvvis	CIS-like excitations from monopole charges, ~1000× faster than full CI
GIAO NMR	spectroscopy::giao_nmr	Multi-nucleus NMR shieldings, complements empirical HOSE

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Retained as Experimental (E4)

EXP-201: Kernel Polynomial Method (KPM)

Feature flag: experimental-kpm Location: experimental/kpm/ (chebyshev.rs, density.rs)

KPM approximates the density of states via Chebyshev expansion of the spectral function. Useful for very large systems (10k+ atoms) but the current implementation lacks:

• Adaptive polynomial order selection
• Sparse-matrix backend for O(N) scaling
• Benchmarks against direct diagonalization for <500 atoms

Promotion criteria:

1. Demonstrate wall-clock advantage over direct DOS for >2000 atoms
2. Validate spectral accuracy within 0.1 eV of full diagonalization
3. Add sparse matrix support (nalgebra-sparse or sprs)

EXP-202: Multi-Basin Hopping (MBH)

Feature flag: experimental-mbh Location: experimental/mbh/ (blocks.rs, hessian.rs)

Block-diagonal Hessian approach for exploring conformational basins. Intended for global optimization of flexible molecules.

Current limitations:

• No energy evaluator integration (needs UFF/MMFF94 coupling)
• Block decomposition heuristic is molecule-size dependent
• Missing Metropolis acceptance criterion

Promotion criteria:

1. Integrate with forcefield module for energy evaluation
2. Reproduce known global minima for test molecules (alanine dipeptide, etc.)
3. Compare basin-hopping yield vs multi-seed ETKDG

EXP-203: Randomized Numerical Linear Algebra (RandNLA)

Feature flag: experimental-randnla Location: experimental/rand_nla/ (nystrom.rs, solver.rs)

Nyström approximation and randomized eigensolvers for large overlap/Fock matrices. Currently a backend alternative, not a user-facing feature.

Current limitations:

• Rank selection requires manual tuning
• No automatic accuracy estimation
• Not yet wired into SCF or EHT solvers

Promotion criteria:

1. Wire into EHT/SCF as optional backend with feature flag
2. Show <1% error in eigenvalues for rank = N/4
3. Demonstrate speedup for N>500 basis functions

EXP-204: Spectral Dimensionality Reduction (SDR)

Feature flag: experimental-sdr Location: experimental/sdr/ (embedding.rs, projections.rs)

Diffusion-map and spectral embedding for chemical space visualization. Alternative to fingerprint-based Tanimoto similarity.

Current limitations:

• Kernel bandwidth selection is fixed
• No out-of-sample extension (Nyström embedding)
• Limited to datasets fitting in memory

Promotion criteria:

1. Validate embeddings preserve known SAR relationships
2. Add adaptive bandwidth selection
3. Compare clustering quality vs Butina (Tanimoto)

EXP-205a: Conformal Geometric Algebra (CGA)

Feature flag: experimental-cga Location: experimental/cga/ (multivector.rs, motor.rs, conformer.rs, materials.rs)

CGA representation of molecular geometry: motors for rigid transforms, conformal points for distance geometry. Research-stage alternative to Cartesian coordinate manipulation.

Current limitations:

• 5D multivector operations have overhead vs 3D rotations
• Conformer pipeline not competitive with ETKDG
• Materials assembly untested with real MOF structures

Promotion criteria:

1. Demonstrate advantage in distance geometry constraint handling
2. Benchmark CGA conformer vs ETKDG quality and speed
3. Show utility for symmetry-constrained crystal assembly

EXP-205b: Growing String Method (GSM)

Feature flag: experimental-gsm Location: experimental/gsm/ (string.rs, saddle.rs)

Reaction path finding via growing string interpolation between reactant and product geometries. Identifies transition states.

Current limitations:

• No gradient source (needs coupled PES evaluation)
• String convergence criteria incomplete
• Saddle point refinement not validated

Promotion criteria:

1. Couple with PM3 or xTB gradient evaluation
2. Reproduce known barrier heights (SN2, Diels-Alder)
3. Compare path energy profiles vs NEB reference

EXP-205c: Continuous Phase Methods (CPM)

Feature flag: experimental-cpm Location: experimental/cpm/ (surface.rs, grand_potential.rs)

Phase-field and grand potential methods for crystal surface thermodynamics and phase boundary determination.

Current limitations:

• Grand potential formulation needs thermodynamic data
• Surface energy calculations untested for ternary systems
• No integration with materials module

Promotion criteria:

1. Validate surface energies against DFT reference for binary alloys
2. Integrate with materials::UnitCell for consistent crystal handling
3. Add Wulff construction for equilibrium crystal shapes