Quantum Chemistry Algorithms

Overview

unitarylab_algorithms.quantum_chemistry provides MolecularDMRGAlgorithm, which always runs this pipeline:

PySCF/OpenFermion electronic structure
  -> physics-tenpy DMRG ground state
  -> SciPy L-BFGS-B CVD circuit conversion
  -> UnitaryLab QWC measurement and OpenQASM export

The algorithm inherits the shared unitarylab_algorithms.algo_base.BaseAlgorithm, returning the standard result dictionary plus energy, active-space, circuit-quality, and measurement fields.

Installation and Platforms

pip install "unitarylab[chemistry]" unitarylab-algorithms

Molecular DMRG and its Chemistry dependencies support Linux and macOS only and do not support Windows. This does not affect the other algorithm categories on Windows. The implementation currently reaches the independent Chemistry package through the compatibility path unitarylab.library.chemistry; Chemistry is not bundled in the main simulator.

Import and Run

MolecularDMRGAlgorithm is included in the root __all__:

from unitarylab_algorithms import MolecularDMRGAlgorithm
 
algorithm = MolecularDMRGAlgorithm(text_mode="plain")
result = algorithm.run(
    xyz_file="H2",
    basis="sto-3g",
    active_occupied_orbitals=1,
    active_virtual_orbitals=1,
    max_iterations=100,
    chi0=20,
    nsweeps=100,
    maxdim=100,
    cutoff=1e-8,
    shots=10_000,
    seed=2077,
    backend="torch",
    device="cpu",
    dtype="complex128",
)
 
print(result["status"])
print(result["DMRG energy"])
print(result["Sampled energy"])

The module-level test() accepts an optional xyz_file and always runs the complete workflow:

from unitarylab_algorithms.quantum_chemistry.molecular_dmrg.algorithm import test
 
test(xyz_file="H2")

The algorithm does not expose the lower-level workflow’s target argument; every run completes electronic structure, DMRG, CVD, and QWC measurement.

Parameters

Bundled molecules are H2, LiH, H2O, CH4, CO, CO2, C6H6, HF, N2, NH3, and O2. Names without extensions resolve under molecules/; external XYZ paths are accepted.

Results and Files

Results include the standard fields status, circuit_path, plot, and circuit, plus these algorithm-specific fields:

• HF energy, Active-space FCI energy, Active space
• DMRG energy, CVD layers, CVD infidelity
• UnitaryLab exact energy, Exact energy difference, Sampled energy
• Measurement groups, Circuit depth, Circuit gates, QASM directory

The default directory is results/quantum_chemistry/molecular_dmrg/ under the current working directory. It contains the BaseAlgorithm result TXT, circuit SVG, and output/ with PySCF data and OpenQASM 2.0 files for each QWC group. The returned plot list contains the TXT and QASM artifacts; QASM entries have their format explicitly corrected to "qasm".

Notes

• Active-space size, DMRG bond dimension, CVD restarts, and optimizer iterations can greatly increase memory and runtime. Start with H2/sto-3g to validate the environment.
• The algorithm retains an active-space FCI reference but does not provide the MP2, CISD, CCSD, full post-SCF FCI switches, or Julia DMRG output from the old implementation.
• MolecularDMRGResult is the result type of the lower-level Chemistry workflow. The algorithm documented here returns a BaseAlgorithm dictionary; do not confuse the two.