Glossary

QM/MM

Quantum mechanics/molecular mechanics. q2m3 treats a small QM region with PySCF and PennyLane. It represents the solvent environment with classical point charges and MM force-field terms.

QPE

Quantum phase estimation. q2m3 uses QPE circuits to estimate molecular energies from a time-evolution phase.

SQD

Sample-based quantum diagonalization. q2m3 prepares a CCSD-seeded LUCJ state in ffsim, samples fixed-particle-number determinants, and solves the Hamiltonian in the recovered subspace with qiskit-addon-sqd.

HF

Hartree-Fock. A mean-field electronic reference. RHF denotes restricted Hartree-Fock for the closed-shell workflows.

CCSD

Coupled cluster with single and double excitations. CCSD(T) adds a perturbative triples correction.

CASCI

Complete active-space configuration interaction. The SQD T0 reference solves the finite active-space Hamiltonian exactly.

SCI

Selected configuration interaction. The calculation retains selected determinants.

AVAS

Atomic valence active space. The fragment workflow uses AO labels to select active orbitals. AO means atomic orbital.

MO

Molecular orbital. Fixed-MO embedding retains the vacuum orbital frame.

LUCJ

Local unitary cluster Jastrow ansatz used by the SQD workflow. Its repetition count and optional spin-channel connectivity are explicit run inputs.

reference tier

The SQD baseline policy: T0 exact active-space CASCI, T1 selected CI, explicitly registered T1+ plugins, or diagnostic T2 CCSD(T). A tier records method and uncertainty semantics. It is not an accuracy label by itself.

EFTQC

Early fault-tolerant quantum computer. q2m3 uses this term for resource-estimation studies such as logical qubit and Toffoli counts.

RDM

Reduced density matrix. q2m3 uses the 1-RDM for Mulliken population analysis and quantum-to-classical observable transfer.

active space

The selected electrons and orbitals included in a quantum calculation. q2m3 documents active spaces as electrons/orbitals and the resulting system qubit count.

Trotterization

Approximation of Hamiltonian time evolution by decomposing it into products of simpler exponentials. Trotter depth affects QPE accuracy and Catalyst compile size.

delta_corr-pol

Correlation-polarization coupling: the change in electronic correlation energy caused by the MM environment.

Catalyst

The PennyLane compilation stack used for @qjit workflows. q2m3 uses it for repeated execution of compiled circuits.

IR cache

A cache of Catalyst intermediate representation artifacts used to avoid repeating expensive compilation stages for unchanged circuit structure.

Hartree

Internal energy unit used by q2m3.

kcal/mol

Reporting unit used only after explicit conversion from Hartree.

Hamiltonian 1-norm

Sum-of-coefficients measure used by the EFTQC resource-estimation path.

MM embedding

Electrostatic coupling of classical point charges into the QM Hamiltonian.

phase wrapping

Ambiguity from measuring phase modulo one. Different energies can map to the same phase at a given evolution time.