std2

Official Resources

• Homepage: https://github.com/grimme-lab/stda
• Source Repository: https://github.com/grimme-lab/stda
• License: GNU General Public License v3.0

Overview

std2 is a software package developed by the Grimme group for performing simplified Time-Dependent Density Functional Theory (sTD-DFT) and simplified Tamm-Dancoff Approximation (sTDA) calculations. These methods provide a highly efficient approximation to full TDDFT, allowing for the computation of ultra-fast UV-Vis absorption and electronic circular dichroism (ECD) spectra for very large molecular systems (thousands of atoms).

Scientific domain: Large-scale excited states, high-throughput screening, UV-Vis/ECD spectroscopy Target user community: Researchers screening large molecules, supramolecular chemists

Theoretical Methods

• Simplified TDA (sTDA)
• Simplified TD-DFT (sTD-DFT)
• Simplified TDA-xTB (sTDA-xTB)
• Tight-binding approximations
• Multipole approximations for integrals
• Point-charge approximations
• Range-separated hybrid functionality

Capabilities (CRITICAL)

• Excitation energies (singlet and triplet)
• Oscillator strengths (UV-Vis)
• Rotatory strengths (ECD)
• High-throughput spectral calculation
• Systems with 1000-5000 atoms
• Full spectral range (Valence/Rydberg)
• Analysis of excitons

Sources: GitHub repository, Grimme group publications

Key Strengths

Extreme Speed:

• Orders of magnitude faster than standard TDDFT
• Seconds/minutes for large molecules
• Integral approximations avoid N^4 scaling

Large Scale:

• Routinely handles >1000 atoms
• Supramolecular complexes
• Protein fragments
• Nanoclusters

Accuracy:

• Errors ~0.2-0.4 eV (comparable to range-separated hybrids)
• Calibrated against high-level methods
• Reliable spectral shapes

Inputs & Outputs

• Input formats:

  • molden file (from ORCA, Turbomole, Gaussian, etc.)
  • xtb output files (for sTDA-xTB)
  • Control input file

• Output data types:

  • Excitation energies
  • Oscillator/Rotatory strengths
  • Simulated spectra (broadened)
  • State character analysis

Interfaces & Ecosystem

• QC Codes (for wavefunction): ORCA, TURBOMOLE, Gaussian, Q-Chem, PSI4 (via Molden)
• xTB: Seamless integration for sTDA-xTB
• Language: Fortran
• Binaries: Static binaries available

Advanced Features

xTB Integration:

• Can run purely on xTB wfn (sTDA-xTB)
• No DFT calculation required
• Extremely fast workflow

Solvent Effects:

• Implicit solvation models accessible via interfaced code
• ALPB (in xTB)

Performance Characteristics

• Speed: Ultra-fast (seconds)
• Accuracy: Qualitative to semi-quantitative
• System size: Up to 5000+ atoms
• Memory: Efficient storage of approximated integrals

Computational Cost

• Wavefunction: Requires ground state (DFT or xTB)
• Excited State: Negligible compared to DFT
• Scaling: N^2 or better with approximations

Limitations & Known Constraints

• Approximation: Not ab initio TDDFT
• Wavefunction dependency: Quality depends on input orbitals
• Charge Transfer: Corrected sTDA can handle it, but check
• Rydberg states: Can be limited by basis set
• Input: Requires Molden file (except xTB mode)

Comparison with Other Codes

• vs Full TDDFT: std2 is approx. 100-1000x faster, less rigorous
• vs ZINDO: std2 generally more robust and accurate
• vs DFTB: Similar niche, std2 uses DFT orbitals
• Unique strength: Unmatched speed for realistic TDDFT-quality spectra of huge variants

Application Areas

• Screening: Calculating spectra for thousands of conformers
• Supramolecular: Host-guest complexes
• Bio-organic: Large chromophores in proteins
• Materials: Optical properties of large aggregates

Best Practices

• Input Orbitals: Use robust DFT functionals (e.g. wB97X-D)
• Basis Set: def2-SVP/TZVP usually sufficient
• TDA vs TDDFT: sTDA usually robust, sTD-DFT includes de-excitation
• Verification: Check a small model with full TDDFT

Community and Support

• Open-source GPL v3
• Grimme group support
• Binaries provided
• Used in xtb ecosystem

Verification & Sources

Primary sources:

1. GitHub: https://github.com/grimme-lab/stda
2. S. Grimme, J. Chem. Phys. 138, 244104 (2013)
3. C. Bannwarth, S. Grimme, Comput. Theor. Chem. 1040, 45 (2014)

Confidence: VERIFIED - Grimme group official code

Verification status: ✅ VERIFIED

• Official homepage: ACCESSIBLE
• Source code: OPEN (GPL v3)
• Method: sTDA / sTD-DFT (Widely cited)
• Specialized strength: Ultra-fast excited states for massive systems