Semiempirical Quantum Mechanics

For semiempirical quantum mechanics (SQM) engines to fit the AtomicInput/Result schema the following convention is used:

• Method: The unique method name (PM7 or GFN2-xTB) including the parametrisation information is provided, no basis is needed

As for quantum mechanical methods a minimal Molecule object is sufficient as input.

Note

Semiemprical engines might not handle the concept of ghost atoms correctly, check carefully how the used engine handles ghost atoms. To be sure remove ghost atoms from input to semiempirical engines beforehand.

Example

For example, running a calculation with the GFN2-xTB method using the xtb engine would work like any other QM engine with

>>> import qcelemental as qcel
>>> mol = qcel.models.Molecule(
...     symbols=["O", "H", "H"],
...     geometry=[
...         [ 0.00000000000000, 0.00000000000000,-0.73578586109551],
...         [ 1.44183152868459, 0.00000000000000, 0.36789293054775],
...         [-1.44183152868459, 0.00000000000000, 0.36789293054775],
...     ],
... )
...
>>> model = qcel.models.AtomicInput(
...     molecule=mol,
...     driver="energy",
...     model={"method": "GFN2-xTB"},
... )
...
>>> import qcengine as qcng
>>> ret = qcng.compute(model, "xtb")
>>> ret.return_result
-5.070451354836705

MOPAC

The following semiempirical Hamiltonians are supported with the MOPAC engine.

Method	Basis
mndo	None
am1	None
pm3	None
rm1	None
mndod	None
pm6	None
pm6-d3	None
pm6-dh+	None
pm6-dh2	None
pm6-dh2x	None
pm6-d3h4	None
pm6-3dh4x	None
pm7	None
pm7-ts	None

xtb

The following extended tight binding Hamiltonians are available with the xtb engine.

Method	Basis	Reference
GFN2-xTB	None	10.1021/acs.jctc.8b01176
GFN1-xTB	None	10.1021/acs.jctc.7b00118
GFN0-xTB	None	10.26434/chemrxiv.8326202.v1