Run OpenQP from Python¶

OpenQP can be driven entirely from Python. The Python API is meant to feel like an OpenQP calculation expressed as a script: define the molecule, choose the quantum theory, choose the workflow when it is not a plain energy calculation, run the calculation, and read results.

The recommended high-level entry point is oqp.openqp.OpenQP. It keeps the native OpenQP section names available for advanced cases, while keeping normal scripts organized around six top-level ideas.

API Shape¶

Top-level call	Purpose
`job.molecule(...)`	Molecular identity: geometry, charge, multiplicity, and optional second geometry.
`job.theory.<model>(...)`	Quantum theory: HF, DFT, MP2, CCSD, CCSD(T), native wavefunction methods, TDHF, TDDFT, SF-TDDFT, MRSF-TDDFT, functional, basis, response states, and reference type.
`job.workflow.*(...)`	Calculation type: gradient, Hessian, optimization, SOC, NACME, EKT, PCM, NMR, NAMD, and related job workflows. Plain energy calculations need no workflow call.
`job.qmmm(...)`	Enable ESPF QM/MM embedding: sets `[input] qmmm_flag=true` and the `[qmmm]` section (PDB, force field, QM atoms, cutoff, embedding).
`job.control(...)`	Hardware and runtime controls such as `usempi` and `omp_threads`.
`job.settings.*(...)`	Specialized detailed settings that are not part of ordinary molecule/theory/workflow setup, such as atom-wise basis assignment.
`job.run()`	Execute the calculation and return the OpenQP `Molecule` result object.

Minimal MRSF-TDDFT Script¶

MRSF-TDDFT is a central OpenQP workflow. The compact helper sets the standard MRSF blocks while leaving the detailed OpenQP keywords available.

from oqp.openqp import OpenQP

job = OpenQP("h2o_mrsf", silent=1)

job.molecule(geometry="water", charge=0)
job.theory.mrsf(functional="bhhlyp", basis="6-31g*", nstate=3)

mol = job.run()
results = mol.get_results()

print("Ground/reference energy:", results["energy"])
print("TD energies:", results["td_energies"])

After an MRSF energy run, the same Python script can continue into excited-state analysis and export through oqp.interop.

from oqp.interop import MRSFExcitedStates, nto_transition

states = MRSFExcitedStates(mol)
nto = nto_transition(states, 0, 1)

print("S0 -> S1 oscillator strength:", states.oscillator_strength(0, 1))
print("Leading NTO weight:", nto["weights"][0])

See MRSF Analysis and Interoperability for NTOs, attachment/detachment densities, cube export, QCSchema, FCIDUMP, and external-code comparisons.

Minimal HF Script¶

from oqp.openqp import OpenQP

job = OpenQP("h2o_hf", silent=1)

job.molecule(
    """
O   0.000000000   0.000000000  -0.041061554
H  -0.533194329   0.533194329  -0.614469223
H   0.533194329  -0.533194329  -0.614469223
""",
    charge=0,
    multiplicity=1,
)
job.theory.hf(basis="6-31g*")

mol = job.run()
print("SCF energy:", mol.get_scf_energy())

Every value can still be overridden through detailed settings when needed.

Minimal DFT Script¶

DFT uses its own helper so HF and Kohn-Sham jobs stay visually distinct.

from oqp.openqp import OpenQP

job = OpenQP("h2o_pbe", silent=1)
job.molecule(geometry="water", charge=0, multiplicity=1)
job.theory.dft(functional="pbe", basis="6-31g*")

mol = job.run()
print("DFT energy:", mol.get_scf_energy())

Minimal MP2 Script¶

MP2 uses an HF reference and is selected with job.theory.mp2(...). RHF supports analytic gradients and gradient-driven geometry calculations; UHF and ROHF remain energy-only.

from oqp.openqp import OpenQP

job = OpenQP("h2o_mp2", silent=1)
job.molecule(geometry="water", charge=0, multiplicity=1)
job.theory.mp2(basis="6-31g", reference="uhf", variant="scs-mp2", conv=1.0e-10)

mol = job.run()
print("MP2 total energy:", mol.get_results()["energy"])

For an analytic spin-scaled RHF-MP2 gradient:

job = OpenQP("h2o_scs_mp2_grad", silent=1)
job.molecule(geometry="water", charge=0, multiplicity=1)
job.theory.mp2(basis="6-31g", reference="rhf", variant="scs-mp2", conv=1.0e-10)
job.workflow.gradient(state=0)

mol = job.run()
print("SCS-MP2 gradient:", mol.get_grad())

Minimal Coupled-Cluster Script¶

Coupled cluster also uses an HF reference and is also energy-only. job.theory.ccsd_t(...) adds the perturbative triples correction; job.theory.ccsd(...) is the same helper without it.

from oqp.openqp import OpenQP

job = OpenQP("h2o_ccsd_t", silent=1)
job.molecule(geometry="water", charge=0, multiplicity=1)
job.theory.ccsd_t(basis="6-31g", reference="rhf", nfzc=1, conv=1.0e-7)

mol = job.run()
print("CCSD(T) total energy:", mol.get_results()["energy"])

nfzc, conv, maxit, and ndiis are the coupled-cluster controls and route to [cc]; the SCF keeps its own conv and maxit, so any other keyword passed here routes to [scf] as usual. See Coupled Cluster for the reference types and their cost.

Theory, Workflow, And Settings¶

The distinction is intentional: theory chooses the quantum model, workflow chooses the kind of calculation to run, and settings holds specialized low-level details.

job = OpenQP("custom_mrsf")

job.molecule("H 0 0 0; H 0 0 0.74", charge=0)
job.theory.mrsf(
    functional="bhhlyp",
    basis="6-31g*",
    nstate=5,
)
job.settings.tdhf(target=2)
job.workflow.gradient(state=3)

mol = job.run()

Ordinary basis selection belongs with the theory:

job.theory.dft(functional="pbe0", basis="def2-svp")
job.theory.mp2(basis="6-31g", reference="uhf", variant="sos-mp2")
job.theory.tddft(functional="b3lyp5", basis="6-31g*", nstate=3)
job.theory.sf_tddft(functional="bhhlyp", basis="6-31g*", nstate=3)

For existing scripts, job.theory("mrsf-tddft", ...) and related string dispatch calls remain supported. New examples prefer job.theory.mrsf(...), job.theory.dft(...), job.theory.mp2(...), and the other model-specific helpers.

For gradients, Python uses state=... even though the input-file keyword is [properties] grad. HF/DFT and RHF-MP2 reference gradients use state=0. Ordinary TDHF/TDDFT state 1 is the first excited state. SF-TDDFT and MRSF-TDDFT state 1 is the lowest spin-flip/MRSF target state, which can be the multiconfigurational ground state.

Specialized atom-wise basis assignment belongs in settings:

job.molecule("Br 0 0 0; H 0 0 1.4", charge=0, multiplicity=1)
job.theory.dft(functional="bhhlyp")
job.settings.basis(["LANL2DZ", "6-31g*"])

For tagged basis libraries, include atom tags in the geometry and map each tag:

job.molecule(
    """
C  0.0  0.0  0.0  c1
H  0.0  0.0  1.0  h1
H  1.0  0.0  0.0  h1
""",
    charge=0,
)
job.theory.mrsf(functional="bhhlyp", nstate=5)
job.settings.basis(c1="cc-pvdz", h1="6-31g*")

Raw section updates remain useful when another program generates the input:

job.settings.input(ispher="auto")
job.settings.scf(conv=1.0e-8)
job.settings.tdhf(target=2)

job.update({
    "input": {"method": "tdhf", "functional": "bhhlyp", "runtype": "energy"},
    "scf": {"type": "rohf", "multiplicity": 3},
    "tdhf": {"type": "mrsf", "nstate": 3},
})

QM/MM and Nonadiabatic Dynamics¶

job.qmmm(...) enables ESPF QM/MM embedding, and job.workflow.namd(...) runs nonadiabatic (surface-hopping) molecular dynamics. They compose with the usual job.molecule(...) / job.theory.*(...) calls, so QM/MM, SOC-NAMD, and SOC-NAMD-QMMM are all built the same way.

job.qmmm(...) sets [input] qmmm_flag=true and the [qmmm] section. The QM region and atom selection come from job.molecule("file.pdb <indices>"); QM/MM molecular dynamics additionally reads the PDB, force field, and QM atoms from job.qmmm(...). forcefield is an alias for the [qmmm] forcefield_files list — a Python list is joined into the comma-separated string OpenQP expects, and qm_atoms accepts a string ("0-2") or a list of indices.

# Single-point QM/MM energy
from oqp.openqp import OpenQP

job = OpenQP("ala_qmmm", silent=1)
job.molecule("ala.pdb 9 10 17 18 19", basis="6-31g*", charge=0)
job.theory("hf", functional="bhhlyp")
job.qmmm(embedding="electrostatic")
mol = job.run()

job.workflow.namd(...) selects runtype=namd and sets the [md] section. Like the other MRSF workflows it requires an MRSF-TDDFT theory. Pass soc=True (with an optional soc_basis) for SOC-NAMD, and add job.qmmm(...) for QM/MM.

# SOC-NAMD-QMMM in a periodic water box
job = OpenQP("chromophore", silent=1)
job.molecule("system.pdb 0-14", basis="6-31g*")
job.theory.mrsf(functional="bhhlyp", nstate=3)
job.qmmm(
    pdb_file="system.pdb",
    forcefield=["amber14-all.xml", "amber14/tip3p.xml"],
    qm_atoms="0-14",
    cutoff="PME",
    embedding="electrostatic",
    rigidwater=True,
)
job.workflow.namd(soc=True, soc_basis="mch", nstep=200, dt=0.5, init_state="S1")
mol = job.run()

For an ensemble, keep one campaign seed and assign a distinct rng_stream to each trajectory. Hopping random numbers are then reproducible functions of the campaign seed, trajectory stream, and physical MD step. By default, seed=0 resolves once to the local YYYYMMDD date, rng_stream=1, and first_hop_step=1. Step 1 already has the initial and propagated structures, so it is the first TLF2 interval and the first electronic-coefficient propagation and hopping decision. Setting first_hop_step=2 explicitly delays only the active-state transition and hopping RNG: step 1 still propagates the coefficients and computes hop probabilities. Same-spin NAMD defaults to nacme_check="baeck_an", which logs an independent energy-curvature estimate beside the overlap/TLF TDC. The comparison is magnitude-only because TD-BA has no wavefunction phase information. The default nacme_gate="off" records the diagnostic without enforcing it; use warn or error only after calibrating the absolute and relative tolerances for the target system. The same gate accepts a signed, phase-aligned analytic d_IJ . v reference when that provider is connected in a later release.

All same-spin/SOC and gas-phase/QM/MM NAMD drivers write a dense appendable, packed-binary <project>.namd.trj and an atomic compressed checkpoint. Canonical .oqp runs additionally write a directly runnable <project>.namd.restart.oqp; Python-API and legacy .inp runs must restart by reusing their original configuration with restart=true and explicit checkpoint and trajectory paths. The trajectory is designed for NumPy memory mapping rather than human reading. Use trajectory_interval to trade temporal resolution for file size. Zero, the default, selects an interval of approximately 10 fs from dt; positive values are step counts. The final point and strict NACME or NVE failures are retained even between regular output points. The human-readable NACME validation table remains in the main log:

from oqp.library.namd import read_namd_trajectory

metadata, trj = read_namd_trajectory("job.namd.trj")
s1_s0_tdc = trj["overlap_tdc_au"][:, 0, 1]
phase_margin = trj["tracking_margin"]

For SOC trajectories, the packed record also contains the complex spin-adiabatic overlap and anti-Hermitian TDC as real/imaginary fields and the active representation. The restart checkpoint preserves the previous SOC eigensystem and singlet/triplet response vectors, validating their exact shapes and dtypes before continuation. TD-Baeck–An remains a same-spin diagnostic and is contextually disabled for SOC.

Dropping job.qmmm(...) gives gas-phase dynamics, and dropping soc=True gives internal-conversion FSSH:

job = OpenQP("gas_namd", silent=1)
job.molecule(geometry="water", charge=0)
job.theory.mrsf(functional="bhhlyp", basis="6-31g*", nstate=2)
job.workflow.namd(nstep=100, dt=0.5, active=1)
mol = job.run()

Nonadiabatic QM/MM dynamics currently supports whole-molecule QM regions; see the SOC-NAMD-QMMM workflow, the [md] section, and the [qmmm] section for the full contract.

Molecular Geometry¶

job.molecule(...) accepts named geometries, inline coordinates, atom lists, file paths, and an optional second geometry for two-structure workflows such as NACME. Keep method, functional, and basis setup in job.theory.<model>(...).

Named geometries keep small setup scripts short:

job.molecule(geometry="water", charge=0, multiplicity=1)
job.molecule(geometry="h2o", charge=0, multiplicity=1)
job.molecule(geometry="ch4", charge=0, multiplicity=1)

OpenQP first checks a small built-in table for common molecules such as water, h2o, h2, ch4, methane, nh3, and co2. If the name is not built in, source="auto" tries PubChem.

job.molecule(geometry="benzene", source="pubchem", charge=0, multiplicity=1)

To inspect or reuse the generated geometry text directly:

from oqp.openqp import get_geometry

system = get_geometry("water")
job.molecule(system, charge=0, multiplicity=1)

Explicit molecular geometries still work as before:

job.molecule("H 0 0 0; H 0 0 0.74", charge=0, multiplicity=1)

job.molecule([
    ("H", 0.0, 0.0, 0.0),
    ("H", 0.0, 0.0, 0.74),
])

job.molecule("h2.xyz", charge=0, multiplicity=1)

Inline coordinates are Angstrom by default. If a script supplies Bohr coordinates, pass unit="Bohr" and OpenQP will convert the geometry text before loading it.

job.molecule("H 0 0 0; H 0 0 1.4", unit="Bohr", charge=0, multiplicity=1)

Two-geometry workflows can pass the second geometry directly:

job.molecule(system, system2, charge=0)

PySCF Conversion Bridge¶

OpenQP does not use PySCF-style constructor keywords as its main scripting language. For mixed workflows, use the explicit conversion bridge:

from oqp.openqp import OpenQP

job = OpenQP.from_pyscf(pyscf_mol, project="mixed_workflow")
job.theory.mrsf(functional="bhhlyp", basis="6-31g*", nstate=5)

mol = job.run()

The bridge reads common PySCF-like molecule attributes (atom, basis, charge, spin, and unit) and translates them into OpenQP sections. After conversion, use normal OpenQP workflow and section calls.

Existing Input Files and Lower-Level API¶

For direct .inp file execution or explicit sectioned dictionaries, use the lower-level Runner API. Those examples are kept in the Python API reference so this manual page stays focused on the compact OpenQP scripting style.

Reading Results¶

job.run() returns the OpenQP Molecule object and also stores it as job.mol.

mol = job.run()

energy = mol.get_scf_energy()
coordinates_bohr = mol.get_system()
gradient = mol.get_grad()
raw_data = mol.get_data()

For orbital and matrix data, OpenQP creates tag-based accessors such as get_dm_a(), get_fock_a(), get_e_mo_a(), get_vec_mo_a(), get_td_energies(), and get_soc_eval() when the corresponding data are available.

Runner gives two convenient result paths.

Access pattern	Use
`runner.results()`	Compact Python summary with atoms, coordinates, energy, gradients, NAC, SOC, and raw data tags.
`runner.mol.get_results()`	JSON-friendly calculation summary similar to OpenQP output records.
`runner.mol` getters	Direct access to selected quantities and native OpenQP data tags.

Threading and MPI¶

Runtime controls belong in job.control(...). For ordinary Python scripts, use usempi=False unless the script is launched in an MPI environment. OpenMP threads can be set in the same call:

job = OpenQP("h2")
job.molecule("H 0 0 0; H 0 0 0.74", charge=0, multiplicity=1)
job.theory.hf(basis="6-31g*")
job.control(usempi=False, omp_threads=8)
job.run()

For large MPI jobs, keep the same Python API but launch the script with the MPI launcher used on the machine.

Validating Inputs¶

Runner validates input before dispatching expensive kernels. Front ends and custom scripts can call the input checker directly when they need a preflight report. See Input Validation for the structured diagnostic API.

For lower-level API details, see Python API and Results and Molecule Data.