Pyscf

PySCF

What this library is for

PySCF is a Python-native electronic-structure package for molecular and periodic quantum chemistry. It provides Gaussian-basis molecule/cell builders, SCF, DFT, MP2, coupled cluster, CI/FCI, CASSCF, TDDFT, gradients, geometry optimization, solvent, QM/MM, and periodic boundary-condition workflows.

When to use this vs. alternatives

• Use PySCF for quantum chemistry calculations where method, basis, charge, spin, SCF convergence, molecular orbitals, density matrices, or post-HF methods are central to the task.
• Use ASE to orchestrate atomistic workflows or connect structures to calculators; convert to PySCF only when the calculation is quantum chemistry in PySCF's method stack.
• Use pymatgen for materials structure/phase analysis; use PySCF PBC modules for electronic-structure calculations on periodic cells.
• Use RDKit for cheminformatics and conformer generation before a quantum chemistry calculation; validate charge, spin, coordinates, and atom order before passing geometries to PySCF.
• Do not implement Hartree-Fock, DFT grids, integrals, or CCSD with generic NumPy/SciPy unless the user is developing a new electronic-structure method.

Canonical workflow

Most calculations follow: define Mole or periodic Cell, choose a method object, call .kernel(), then inspect convergence and derived quantities.

from pyscf import cc, dft, gto, mp, scf

mol = gto.M(
    atom="""
    O  0.000000  0.000000  0.000000
    H  0.000000  0.757160  0.586260
    H  0.000000 -0.757160  0.586260
    """,
    basis="cc-pvdz",
    charge=0,
    spin=0,  # 2S = n_alpha - n_beta
    unit="Angstrom",
    verbose=4,
)

mf = scf.RHF(mol)
e_hf = mf.kernel()
assert mf.converged

ks = dft.RKS(mol)
ks.xc = "b3lyp"
e_dft = ks.kernel()

mp2 = mp.MP2(mf)
e_corr, t2 = mp2.kernel()

mycc = cc.CCSD(mf)
e_ccsd = mycc.kernel()[0]

print("E_HF", e_hf)
print("E_DFT", e_dft)
print("E_MP2_total", e_hf + e_corr)
print("E_CCSD_total", e_hf + e_ccsd)

For deeper examples, read:

• Quickstart: https://pyscf.org/quickstart.html
• User guide: https://pyscf.org/user/index.html
• How to use PySCF: https://pyscf.org/user/using.html
• Examples: https://github.com/pyscf/pyscf/tree/master/examples

Key conventions and gotchas

• spin is 2S, equal to n_alpha - n_beta, not multiplicity. A triplet has spin=2, not spin=3.
• Molecular coordinates are commonly given in Angstrom; set unit explicitly when generating geometries from other packages.
• If you mutate a Mole object's attributes after construction, call build() again before running a calculation.
• Closed-shell systems normally use RHF/RKS; open-shell systems require UHF/UKS or ROHF/ROKS as appropriate. Do not run RHF on a radical because it happens to converge.
• Basis set and ECP/pseudopotential choices define the calculation. Record basis, ECP/pseudo, charge, spin, XC functional, frozen-core choices, and density-fitting settings.
• DFT energies depend on XC functional and numerical grid. Tighten grids for sensitive energies, nonlocal corrections, or reproducibility comparisons.
• Post-HF methods should start from a converged and appropriate mean-field reference; inspect spin contamination for unrestricted references.
• Periodic calculations use pyscf.pbc cells, lattice vectors, pseudopotentials, density fitting, and k-points. Do not treat a periodic material as a large molecule unless that is the intended approximation.

Anti-patterns

• Do not guess spin from chemical formula alone. Determine charge, electron count, multiplicity, and whether restricted/open-shell methods are appropriate.
• Do not compare energies across different basis sets, ECPs, grids, frozen-core settings, charge/spin states, or geometries as if they are one calculation.
• Do not ignore mf.converged or SCF warnings. Try better initial guesses, damping, level shifting, Newton SCF, density fitting, or a more suitable reference before reporting results.
• Do not pass RDKit/ASE/pymatgen coordinates into PySCF without checking units, atom order, total charge, spin state, and whether hydrogens/protons are explicit.
• Do not use molecular gto.M for a periodic cell that needs k-points and lattice vectors; use pyscf.pbc.gto.Cell.

Diagnostic checks

Before trusting outputs, the agent should:

• Print method, basis, charge, spin, unit, electron count, atom count, and coordinates or geometry source.
• Check SCF convergence, total energy, HOMO/LUMO or occupations, and warnings.
• For unrestricted calculations, inspect spin expectation or spin contamination where relevant.
• For DFT, record XC functional, grids, dispersion/nonlocal settings, and integration-grid changes.
• For correlated methods, record frozen-core settings, reference type, and whether amplitudes/convergence are sane.
• For PBC, record lattice vectors, pseudopotentials, k-point mesh, density fitting, and whether all-electron or pseudopotential treatment is used.

Pointers to deeper material

• Documentation: https://pyscf.org/
• Quickstart: https://pyscf.org/quickstart.html
• User guide: https://pyscf.org/user/index.html
• Source repository: https://github.com/pyscf/pyscf
• Paper: Sun et al. (2018), "PySCF: the Python-based simulations of chemistry framework", WIREs Computational Molecular Science 8, e1340. https://doi.org/10.1002/wcms.1340

Optimization Candidate Notes

• For open-shell prompts, make the spin convention impossible to miss: PySCF spin is 2S = n_alpha - n_beta, so multiplicity is spin + 1; choose UHF/UKS or ROHF/ROKS instead of RHF/RKS for radicals and other open-shell systems.
• For geometry optimization prompts, use the documented PySCF gradient/optimizer path (pyscf.geomopt.geometric_solver.optimize, berny_solver.optimize, or method .Gradients() as appropriate) rather than generic scipy.optimize.
• Keep examples compact but always print convergence, total energy, charge, spin, basis, method/functional, final coordinates, and <S^2>/spin contamination when an unrestricted reference is used.