liminfoGaussian Reference
Free reference guide: Gaussian Reference
About Gaussian Reference
The Gaussian Reference is a quick-lookup guide for Gaussian computational chemistry software input file syntax and keywords. It covers the major electronic structure methods: Hartree-Fock (HF), density functional theory methods including B3LYP, wB97XD (dispersion-corrected), and M06-2X (Minnesota functional), post-Hartree-Fock methods like MP2 (second-order Moller-Plesset perturbation theory) and CCSD(T) (the gold standard coupled cluster method with perturbative triples).
The reference organizes Gaussian keywords into five practical categories: Methods (HF, DFT, MP2, CCSD(T)), Job Types (Opt for geometry optimization, Freq for vibrational analysis and thermodynamics, SP for single-point energy, IRC for intrinsic reaction coordinate, TD-DFT for excited states and UV-Vis spectra, NBO for natural bond orbital analysis, PCM solvation), Basis Sets (Pople sets like 6-31G(d,p) and 6-311+G(2d,p), Dunning correlation-consistent sets cc-pVDZ/TZ/QZ, Ahlrichs def2-SVP/TZVP, and LanL2DZ ECP for transition metals), Input Settings (%chk, %mem, %nproc, charge/multiplicity), and Troubleshooting (SCF convergence, transition state optimization, Guess=Read).
Designed for computational chemists, graduate students, and researchers running Gaussian calculations, this reference provides the exact route line syntax, option combinations, and practical tips for each keyword. Whether you are optimizing molecular geometries, computing reaction energetics with IRC, predicting UV-Vis spectra with TD-DFT, or analyzing bonding with NBO, the reference gives you the input syntax you need without searching through the full Gaussian documentation.
Key Features
- Covers electronic structure methods from HF through DFT (B3LYP, wB97XD, M06-2X) to post-HF methods (MP2, CCSD(T)) with route line syntax
- Documents all major job types: Opt (with Tight, CalcFC, TS options), Freq (ZPE, enthalpy, Gibbs energy, IR spectrum), SP, IRC, TD-DFT, NBO, and PCM solvation
- Comprehensive basis set reference: Pople (6-31G to 6-311+G), Dunning (cc-pVDZ to aug-cc-pVQZ), Ahlrichs (def2-SVP to def2-QZVP), and LanL2DZ ECP with mixed basis syntax
- Input file configuration: %chk checkpoint files, %mem memory allocation, %nproc CPU cores, and charge/multiplicity specification for neutrals, ions, and open-shell systems
- Transition state optimization syntax with Opt=(TS,CalcFC,NoEigenTest) and verification via imaginary frequency analysis
- SCF convergence troubleshooting: SCF=QC, SCF=XQC, and SCF=(QC,MaxCycle=200) for difficult electronic structures
- Sequential calculation strategy with Guess=Read and Geom=Check for small-to-large basis set energy refinement
- Solvation modeling with SCRF=(PCM,Solvent=Water) and SCRF=(SMD,Solvent=Water) for implicit solvent effects
Frequently Asked Questions
What computational methods does this Gaussian reference cover?
The reference covers Hartree-Fock (HF), DFT functionals including B3LYP, wB97XD (with dispersion correction for non-covalent interactions), and M06-2X (Minnesota functional), as well as post-Hartree-Fock methods MP2 (second-order perturbation theory) and CCSD(T) (coupled cluster with perturbative triples, the gold standard for accuracy). Each entry shows the exact route line syntax such as #p B3LYP/6-311+G(d,p) Opt Freq.
How do I choose the right basis set for my Gaussian calculation?
For quick preliminary results, use 6-31G(d) (double-zeta with polarization). For publication-quality DFT, use 6-311+G(d,p) or def2-TZVP (triple-zeta). For high-accuracy post-HF methods, use Dunning cc-pVTZ or cc-pVQZ sets. Add diffuse functions (+, aug-) for anions and weak interactions. For transition metals, use LanL2DZ effective core potential or mixed basis (GenECP) combining all-electron basis for light atoms with ECP for heavy atoms.
What is the difference between Opt, Freq, and SP job types?
Opt performs geometry optimization to find the energy minimum (or saddle point with Opt=TS). Freq performs vibrational frequency analysis to compute zero-point energy, enthalpy, Gibbs free energy, and predict IR spectra. SP (Single Point) calculates the energy at a fixed geometry without optimization, often used to refine energies at a higher theory level after optimizing at a lower level. A typical workflow is: Opt at B3LYP/6-31G(d), then SP at CCSD(T)/cc-pVTZ.
How do I set up a transition state search in Gaussian?
Use Opt=(TS,CalcFC,NoEigenTest) in the route line. TS tells Gaussian to search for a first-order saddle point. CalcFC calculates the initial Hessian (force constants) for better convergence. After optimization, run a Freq calculation to verify that exactly one imaginary (negative) frequency exists, corresponding to the reaction coordinate. Use IRC=(CalcFC,MaxPoints=50) to trace the intrinsic reaction coordinate from the TS to reactant and product.
What does the charge and multiplicity line mean?
The charge-multiplicity line (e.g., 0 1) specifies the molecular charge and spin multiplicity. Common settings: 0 1 for a neutral singlet, 1 1 for a +1 cation singlet, -1 1 for a -1 anion singlet, 0 2 for a neutral doublet radical, and 0 3 for a neutral triplet. Multiplicity = 2S+1 where S is the total spin quantum number. Open-shell systems (multiplicity > 1) require unrestricted methods (UB3LYP, UHF).
How do I fix SCF convergence problems in Gaussian?
Start with SCF=QC (quadratically convergent SCF). If that fails, try SCF=XQC which attempts normal SCF first then switches to QC. Increase iterations with SCF=(QC,MaxCycle=200). Other strategies include using a smaller basis set first and reading the wavefunction with Guess=Read, changing the initial guess with Guess=Huckel or Guess=Mix, or using level shifting with SCF=(Shift,MaxCycle=200).
How do I model solvent effects in Gaussian?
Use SCRF=(PCM,Solvent=Water) for the Polarizable Continuum Model, which treats the solvent as a dielectric continuum around a molecular cavity. For more accurate solvation free energies, use SCRF=(SMD,Solvent=Water) which includes non-electrostatic contributions. Available solvents include Water, DMSO, Ethanol, THF, and many others. Combine with Opt to optimize geometry in solution.
What memory and CPU settings should I use?
Set memory with %mem (e.g., %mem=8GB). The default 256MB is usually insufficient; most calculations need 4-16GB, and large molecules or high-level methods may need 32-64GB. Set CPU cores with %nproc (e.g., %nproc=16). Parallel efficiency varies by job type: DFT scales well, but CCSD(T) may not scale linearly. Always save checkpoint files with %chk for restart capability, and convert with formchk for orbital visualization.