Basis sets for AICCM

Both CCM lines select their orbital and auxiliary bases through the same vibe-qc basis-set machinery, but which basis you need depends on the backend, the system, and the reference you are targeting.

Orbital basis

The orbital basis labels are the same as everywhere else in vibe-qc: built-in names (sto-3g, 6-31g*, cc-pvdz) or pob-* solid-state bases (pob-tzvp-rev2, pob-dzvp-rev2).

basis

purpose

Δ vs CRYSTAL

sto-3g

Cross-check. Cheap, well-conditioned, works for every element H–Kr. The default for the AICCM-2026 benchmark so that Γ-CCM, χ-CCM, bipole, and GDF all run on the same basis.

large (minimal basis), not for production

pob-tzvp-rev2

Production comparison. The solid-state triple-zeta basis used in the CRYSTAL23 .d12 library. Run the periodic references (bipole/gdf) at this basis to compare against CRYSTAL23 directly.

~mHa/atom for insulators

pob-dzvp-rev2

Lighter solid-state double-zeta; faster SCF convergence than TZVP.

larger than TZVP, smaller than STO-3G

6-31g*

Molecular basis; usable for 1-D chains and vacuum-padded slabs.

N/A for bulk

Note

The Γ-CCM test set uses sto-3g by default for the four-center routes and pob-tzvp-rev2 for the periodic reference routes (via --crystal-basis). The χ-CCM test set uses sto-3g and specifies the auxiliary basis explicitly.

Specifying the orbital basis

Γ-CCM test-set runner:

python run_case.py c-diamond aiccm-hf --basis pob-tzvp-rev2
python run_case.py mgo        gdf     --basis pob-tzvp-rev2   # CRYSTAL23 comparison

χ-CCM test-set runner:

python run_case_b.py mgo rhf-ri --basis pob-tzvp-rev2

Python API:

# Γ-CCM
ccm = CCMSystem(system, (2, 2, 2), "pob-tzvp-rev2")

# χ-CCM
basis = vq.BasisSet(system.unit_cell_molecule(), "pob-tzvp-rev2")

Auxiliary basis (RI / density fitting)

The auxiliary basis is only needed for RI, RIJCOSX, and GDF routes. The four-center backends need no auxiliary basis.

Γ-CCM

route

auxiliary basis

how it is chosen

aiccm-hf, aiccm-ks

none

four-center, direct

aiccm-ri, aiccm-ks-ri

implicit (GDF auto-builds the auxiliary from the orbital basis)

no user control — the GDF driver constructs the JK-fit basis

aiccm-rijcosx

implicit (RI-J uses GDF; COSX-K uses a grid, no auxiliary)

same as above

ccm_neutral_cderi

you supply the plane-wave cutoff ke_cutoff (Ha)

ccm_neutral_cderi(ccm, ke_cutoff=200.0)

The GDF container auto-selects the auxiliary basis for each element. For the neutral cderi the PW cutoff controls accuracy — the default 120 Ha is converged for STO-3G; for pob-TZVP use 200–300 Ha.

χ-CCM

backend

auxiliary basis

flag

four_center

none

—

ri

def2-svp-jk (default) or any JK-fit basis

--aux-basis def2-svp-jk

rijcosx

same RI-J auxiliary + COSX grid

--aux-basis def2-svp-jk

 
# χ-CCM — explicitly set the auxiliary basis
python run_case_b.py mgo rhf-ri --aux-basis def2-svp-jk
python run_case_b.py mgo rhf-ri --aux-basis def2-tzvp-jk

The χ-CCM fleet runner (make_jobs_b.py) hardcodes --aux-basis def2-svp-jk on every emitted command — the fastest JK-fit basis that still gives ∼sub-mHa accuracy.

In the Python API:

result = vq.run_periodic_job(
    system, basis,
    method="RHF", jk_method="aiccm2026dev-b",
    aiccm_backend="ri",
    aux_basis="def2-tzvp-jk",
    ...
)

GDF settings (χ-CCM)

χ-CCM’s RI/GDF route uses the range-separated GDF (RSGDF) method with a plane-wave cutoff:

python run_case_b.py mgo rhf-ri --gdf-method rsgdf --rsgdf-ke-cutoff 200

The default is rsgdf with ke_cutoff=200 Ha in the fleet runner. Higher cutoff = more accurate RI fit, more expensive.

Basis-set caveats

Cs has no STO-3G. The cscl test-set system defaults to pob-tzvp-rev2 for this reason. The element coverage of STO-3G stops at Kr (Z=36); Cs (Z=55) and heavier elements need a pob-* or other compiled basis.

pob- and linear dependence.* The pob-* bases are designed to avoid linear dependence in crystals (they omit the most diffuse functions). They are the recommended choice for solid-state work. Molecular bases (cc-pVZ, 6-31G) can become linearly dependent in tight crystals — vibe-qc’s linear-dependence removal will catch this, but the removed functions reduce variational freedom.

Basis-set superposition error (BSSE). STO-3G has large BSSE; do not use it for binding energies or adsorption energies. The TZVP bases reduce BSSE to chemically acceptable levels (~1–2 kcal/mol).

See also