Experimental

vibe-qc ships a handful of features behind an experimental gate. They are reachable, but not yet production-certified and subject to change without the usual deprecation cycle. Each one emits an ExperimentalWarning when used.

What’s in this section

• The feature catalog is the complete table of every gated feature, its opt-in, its caveat, and its quantitative-status column. Start there if you want to know what’s gated and why.

• Basis set development, developer documentation for the basis-set optimiser toolchain, design notes, and verification reports.

• AICCM, the ab-initio cyclic cluster model, both the Γ-CCM line (aiccm2026dev-a: Γ-CCM is the neutral finite-torus construction with the real-gamma and neutral-bloch variants, and the four-center variant is the 2014 union-and-weight/Wigner-Seitz lineage kept in the same line) and χ-CCM[1] (aiccm2026dev-b, finite-translation-group characters). Every AICCM variant is experimental; the label vocabulary (production, preview, experimental, gated) that every status page uses is defined in Methods: theory and status.

Basis set development

Basis set development

• Basis set development

AICCM: ab-initio cyclic cluster model

The cyclic cluster model is vibe-qc’s signature real-space approach to periodic systems. Two independent implementations are maintained while the common limit is being established:

• aiccm2026dev-a, the Γ-CCM development line (vibeqc.periodic.ccm). Γ-CCM is the neutral finite-torus construction with two producers, variant="real-gamma" and variant="neutral-bloch"; the four-center variant is the 2014 union-and-weight/Wigner-Seitz lineage kept in the same line. The line’s methods are HF, KS, MP2, UMP2, and CCSD(T), with analytic gradients and derivable properties. The same namespace contains neutral fitted-torus UCCSD(T) and DLPNO controls that D90 leaves without an assigned Γ-CCM or χ-CCM identity.

• aiccm2026dev-b, χ-CCM, the finite-character (Γ-centred character-mesh) CCM line (vibeqc.periodic.chi). It uses the explicit Γ-centred character net, with 3D RHF/RKS/UHF/UKS plus RI-MP2 and local-PNO CCSD(T). Every 1D/2D absolute-energy backend fails closed.

Open-shell capabilities across both lines are documented in Open-shell AICCM.

AICCM reference

• AICCM - Γ-CCM, the aiccm2026dev-a line (experimental)
• AICCM aiccm2026dev-a follow-on - derivation document
• AICCM real-Gamma production track
• χ-CCM / aiccm2026dev-b (experimental)
• Experimental chi fitted Coulomb support
• Experimental chi same-source RHF/UHF CCSD and triples
• Experimental chi translation local MP2
• Experimental chi ROHF and Knowles RMP2
• Experimental chi scalar semilocal ECPs
• Experimental chi checkpoint and restart
• χ-CCM / aiccm2026dev-b: an independent finite-torus derivation
• χ-CCM / aiccm2026dev-b decisions and open questions

AICCM tutorials

• The cyclic cluster model (CCM)
• AICCM quickstart
• Γ-CCM / aiccm2026dev-a: step by step
• χ-CCM / aiccm2026dev-b: 3D backends and dimensional guards
• Experimental χ-CCM occupied localization
• H-chain BvK equivalence: vibe-qc’s CCM ancestor
• The equidistant H chain: a 1D metal on a cyclic ring

AICCM experimental

• AICCM route support matrix
• χ-CCM PAO and PNO pipeline
• χ-CCM finite-torus space-group symmetry
• Diamond Γ-CCM / χ-CCM bond and band comparison
• Open-shell AICCM
• Comparing Γ-CCM and χ-CCM
• Basis sets for AICCM
• AICCM visualization
• AICCM troubleshooting and diagnostics

AICCM examples

The runnable examples live under examples/periodic/:

• aiccm2026dev_a_demo.py, Γ-CCM line, four-centre union-and-weight stack: 8-fold ERI symmetry check on 1-D/2-D/3-D lattices and the HF→MP2→CCSD(T) correlation ladder. It also reports the neutral four-center Madelung-background diagnostic as a separate control.

• aiccm2026dev_b_demo.py, χ-CCM: exercises all three ER backends in 3D; 1D/2D invocations demonstrate the intentional fail-close and do not return absolute energies.

• aiccm2026dev_b_mp2.py, canonical RI-MP2 in 3D.

• aiccm2026dev_b_local_correlation.py, DLPNO-MP2 and DLPNO-CCSD(T) on the χ-CCM finite torus.

• aiccm2026dev_diamond_bonds_bands_compare.py, side-by-side diamond HF/KS property bundles and localized-orbital QVF archives; it records the cross-approach comparison as not-defined and emits no delta.

• benchmark_aiccm2026dev_b.py, H4 convergence comparison against the historical and Γ-CCM weights.

The B/CRYSTAL fleet and future Γ-CCM/χ-CCM study inputs live under studies/aiccm-2026/.

AICCM documentation map

you want to…	read this
Run a minimal example	Quickstart
Run your first Γ-CCM calculation	Γ-CCM tutorial
Run your first χ-CCM calculation	χ-CCM tutorial
See all Γ-CCM methods and routes	Γ-CCM reference
See all χ-CCM methods, backends, and caveats	χ-CCM user guide
See which route supports HF, DFT, MP2, DLPNO, or analytic gradients	Route support matrix
Compare Γ-CCM UHF/UMP2, neutral-control UCCSD(T), and χ-CCM open-shell APIs	Open-shell AICCM
Study Γ-CCM and χ-CCM side by side (no current approach delta)	Cross-stream comparison
Choose the right basis set	Basis sets for AICCM
Debug a failing calculation	Troubleshooting
Generate orbital/density visualizations	Visualization
Understand the broad CCM concept	CCM tutorial
Understand the χ-CCM derivation	χ-CCM derivation
Understand the theory (Γ-CCM paper)	Archived Γ-CCM theory paper

See also

• Roadmap, the v2.x CCM track and where this work fits.

• Troubleshooting, known bugs and workarounds.

• The Archived comparative manuscript, detailed theory record for the Γ-CCM and χ-CCM streams.

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[1]

Γ-CCM and χ-CCM are distinct CCM construction approaches: Γ-CCM is the neutral finite-torus construction with two producers, variant="real-gamma" and variant="neutral-bloch" (ruling R1, 2026-08-21), while the four-center variant is the 2014 union-and-weight/Wigner-Seitz lineage kept in the same aiccm2026dev-a development line; χ-CCM is the finite-translation-group character construction. A declared common exchange-q=0 convention is a comparison constraint, not an identification of the constructions. Their distinction is not a choice of Coulomb kernels, and equality for a specified operator and route must be established rather than inferred from the labels.