qed-tddft

Official Resources

• Homepage: https://github.com/cc-ats/qed-tddft
• Source Repository: https://github.com/cc-ats/qed-tddft
• Documentation: README with examples
• License: Open Source

Overview

qed-tddft is a specialized Python package for Quantum-Electrodynamical Time-Dependent Density Functional Theory (QED-TDDFT) within Gaussian atomic basis sets. It enables the simulation of molecules strongly coupled to quantized electromagnetic field modes in optical cavities, capturing light-matter interactions at the quantum level. Built on top of PySCF, it provides a framework for cavity QED calculations in molecular systems.

Scientific domain: Cavity QED, polaritonic chemistry, strong light-matter coupling
Target user community: Researchers in quantum optics, polaritonic chemistry, and cavity-modified molecular properties

Theoretical Methods

• Time-Dependent Density Functional Theory (TDDFT)
• Quantum Electrodynamics (QED) coupling
• Pauli-Fierz Hamiltonian (TDDFT-PF)
• Gaussian atomic basis sets
• Cavity photon modes
• Light-matter coupling tensors
• Analytic energy gradients

Capabilities

• Ground-state DFT with cavity coupling
• QED-TDDFT excited states
• Polaritonic state calculations
• Cavity frequency specification
• Cavity mode direction control
• Multiple excited state roots
• Analytic gradients for geometry optimization
• Integration with PySCF workflows

Key Strengths

Cavity QED Implementation:

• Full Pauli-Fierz Hamiltonian
• Multiple cavity modes
• Tunable coupling strength
• Arbitrary photon frequencies

PySCF Integration:

• Leverages PySCF infrastructure
• Compatible with all PySCF basis sets
• Uses PySCF SCF methods
• Standard Python workflow

Published Methodology:

• J. Chem. Phys. 155, 064107 (2021)
• J. Chem. Phys. 156, 124104 (2022)
• Peer-reviewed implementation

Gradient Capability:

• Analytic energy gradients
• Geometry optimization in cavities
• Polaritonic potential energy surfaces

Inputs & Outputs

• Input formats:

  • PySCF molecule objects
  • Standard basis set specifications
  • Cavity frequency arrays (NumPy)
  • Cavity mode vectors (NumPy)
  • XC functional specification

• Output data types:

  • Polaritonic excitation energies
  • Oscillator strengths
  • Cavity-matter coupling analysis
  • Gradients for optimization

Interfaces & Ecosystem

• Core dependency:

  • PySCF (required)
  • NumPy for array operations

• Workflow integration:

  • Standard PySCF RKS/UKS objects
  • Any PySCF-supported XC functional
  • Any PySCF-supported basis set

Example Usage

from pyscf import gto, scf
import qed

mol = gto.Mole()
mol.atom = '''H 0 0 0; H 0 0 0.74'''
mol.basis = 'cc-pVDZ'
mol.build()

mf = scf.RKS(mol)
mf.xc = "b3lyp"
mf.kernel()

cavity_freq = numpy.asarray([0.200])
cavity_mode = numpy.asarray([[0.001, 0.0, 0.0]])

cav_model = qed.PF(mf, cavity_mode=cavity_mode, cavity_freq=cavity_freq)
td = qed.TDDFT(mf, cav_obj=cav_model)
td.nroots = 5
td.kernel()

Performance Characteristics

• Speed: Depends on PySCF performance
• System size: Medium molecules (standard TDDFT limits)
• Memory: Standard TDDFT memory requirements
• Scalability: Single-node calculations

Limitations & Known Constraints

• Cavity modes: Single or few modes typical
• Coupling regime: Strong coupling focus
• Relativistic: Non-relativistic only
• Periodic: Molecular systems only
• Platform: Requires PySCF installation

Comparison with Other Codes

• vs standard TDDFT: Adds cavity QED coupling
• vs OpenMolcas QED-CASSCF: Different theory level (DFT vs multi-reference)
• vs Molpro cavity: Open-source alternative
• Unique strength: Gaussian basis QED-TDDFT with gradients

Application Areas

Polaritonic Chemistry:

• Cavity-modified reaction rates
• Polaritonic potential energy surfaces
• Ground state modification under strong coupling

Optical Cavities:

• Molecule-cavity interactions
• Purcell effect simulations
• Cavity-induced energy splittings

Spectroscopy:

• Modified absorption spectra
• Cavity-dressed molecular states
• Light-matter hybridization

Best Practices

• Start with known cavity frequencies
• Test coupling strength convergence
• Compare with cavity-free TDDFT
• Use appropriate XC functionals

Community and Support

• Open-source on GitHub (cc-ats organization)
• Published methodology with references
• Python 100%
• Academic development

Verification & Sources

Primary sources:

1. GitHub repository: https://github.com/cc-ats/qed-tddft
2. J. Yang et al., J. Chem. Phys. 155, 064107 (2021)
3. J. Yang et al., J. Chem. Phys. 156, 124104 (2022)

Confidence: VERIFIED - Published methodology with active GitHub

Verification status: ✅ VERIFIED

• Source code: OPEN (GitHub)
• Documentation: README with examples
• Academic citations: 2 J. Chem. Phys. papers
• Purpose: Research (cavity QED)
• Language: Python 100%