gaussian

name	gaussian
description	Generate and manage Gaussian calculations. Use when the user requests Gaussian, G16, GJF files, or needs hybrid functionals (B3LYP), MP2, CCSD(T), or molecular quantum chemistry with Gaussian basis sets.
compatibility	Requires a valid Gaussian license and installation on the HPC target. Gaussian is commercial software — never distribute binaries or bypass license checks.

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