Gaussian

When to Use

• User explicitly requests Gaussian, G16, G09
• User needs molecular quantum chemistry (not periodic solids — use VASP/QE for that)
• User wants hybrid functionals (B3LYP, PBE0, M06-2X) for molecules
• User needs post-HF methods: MP2, CCSD(T), CBS extrapolation
• User wants transition state search with QST2/QST3
• User requests GJF/COM input file generation

Prerequisites

1. Gaussian (g16/g09) accessible on HPC with valid license
2. Sufficient memory and scratch space (Gaussian is memory-intensive)
3. Structure loaded in viewer — verify with catgo_view(action="get_state")

Workflow Steps

1. Verify structure

catgo_view(action="get_state")

2. Create workflow

catgo_workflow_engine(action="create", params={"name": "Gaussian B3LYP opt+freq"})

3. Add Gaussian task via shell

CatGo does not yet have a native Gaussian engine. Use task_type: "shell".

catgo_workflow_engine(action="add_task", params={
  "workflow_id": "wf_xxx",
  "task_type": "shell",
  "name": "g16_opt",
  "command": "g16 < input.gjf > output.log 2>&1",
  "input_files": {
    "input.gjf": "<GJF content>"
  },
  "system_name": "caffeine_opt"
})

When a @register_engine("gaussian") is added, use task_type: "geo_opt" with software: "gaussian".

Input File Template — Optimization + Frequencies

%nproc=16
%mem=32GB
%chk=checkpoint.chk
# opt freq b3lyp/6-311+g(d,p) empiricaldispersion=gd3bj
  scf=tight int=ultrafine

Title: Geometry optimization with frequency analysis

0 1
C    0.000000    0.000000    0.000000
H    0.000000    0.000000    1.089000
H    1.026719    0.000000   -0.363000
H   -0.513360   -0.889165   -0.363000
H   -0.513360    0.889165   -0.363000

Important: The blank line after coordinates is mandatory. A second blank line terminates the input.

Input File Structure

%nproc=<cores>                    # Link 0 commands (resources)
%mem=<memory>
%chk=<checkpoint_file>
# <method>/<basis> <keywords>     # Route section

<title>                           # Title (free text)

<charge> <multiplicity>           # Charge and spin
<element> <x> <y> <z>            # Cartesian coordinates
                                  # Blank line = end of molecule

Parameter Guidance

Parameter	Typical value	Notes
Method	B3LYP, PBE0, M06-2X, wB97XD	Hybrid DFT for molecules
Basis	6-31G(d), 6-311+G(d,p), def2-TZVP	Pople or Karlsruhe basis sets
%mem	4-64 GB	Gaussian stores integrals in memory
%nproc	8-32	Shared-memory parallel
Dispersion	EmpiricalDispersion=GD3BJ	Add for non-covalent interactions
int	UltraFine	Integration grid; always use for DFT
scf	Tight	SCF convergence; XTight for frequencies

Common Calculation Types

Keyword	Purpose
opt	Geometry optimization
freq	Vibrational frequencies (must be at a stationary point)
opt freq	Optimize then frequencies in one job
opt=(ts,calcfc,noeigen)	Transition state search
opt=(qst2)	TS search from reactant+product geometries
irc=(calcfc,maxpoints=50)	Intrinsic reaction coordinate
td=(nstates=10)	TD-DFT excited states
nmr	NMR chemical shifts
pop=nbo	Natural bond orbital analysis

Solvent Effects

Add implicit solvation:

# opt freq b3lyp/6-311+g(d,p) scrf=(smd,solvent=water)

Common Pitfalls

1. Missing blank lines — Gaussian input requires blank lines between sections and after coordinates. Missing them causes cryptic errors.
2. Wrong multiplicity — open-shell systems need correct spin (2S+1). Doublet radicals: multiplicity=2.
3. %mem too low — Gaussian crashes with "galloc: could not allocate memory". Set %mem to ~80% of available RAM.
4. Basis set not available — not all basis sets cover all elements. Heavy elements need ECP (e.g., LANL2DZ).
5. Frequencies at non-stationary point — freq requires a fully optimized geometry. Run opt freq together.
6. License restrictions — never share Gaussian binaries. Ensure the HPC has a valid site license.
7. Checkpoint file not saved — always use %chk for restart capability and property extraction
8. Forgetting dispersion — B3LYP without GD3/GD3BJ underestimates non-covalent interactions