ORCA Frequency Calculation Skill

When to Use

Use this skill when the user wants to:

• Compute vibrational frequencies of a molecule
• Get an IR spectrum
• Calculate zero-point energy (ZPE)
• Obtain thermochemical quantities (enthalpy, entropy, Gibbs free energy)
• Verify a transition state (exactly one imaginary frequency)
• Confirm a minimum (no imaginary frequencies)

Prerequisites

The input structure MUST be optimized at the same level of theory used for the frequency calculation. Running frequencies on an unoptimized structure will produce meaningless imaginary frequencies.

MCP Tool Examples

Basic frequency calculation

catgo_workflow_engine(action: "create", params: {
  name: "Water frequencies B3LYP"
})

catgo_workflow_engine(action: "add_task", params: {
  workflow_id: "<wf_id>",
  task_type: "freq",
  params: {
    software: "orca",
    orca_method: "B3LYP",
    orca_basis: "def2-SVP",
    charge: 0,
    multiplicity: 1
  }
})

Opt + Freq chain (recommended workflow)

Always optimize first, then run frequencies on the result:

catgo_workflow_engine(action: "add_task", params: {
  workflow_id: "<wf_id>",
  task_type: "geo_opt",
  task_id: "opt1",
  params: {
    software: "orca",
    orca_method: "B3LYP",
    orca_basis: "def2-TZVP",
    orca_extra_keywords: "TightOpt D3BJ",
    charge: 0,
    multiplicity: 1
  }
})

catgo_workflow_engine(action: "add_task", params: {
  workflow_id: "<wf_id>",
  task_type: "freq",
  task_id: "freq1",
  depends_on: ["opt1"],
  params: {
    software: "orca",
    orca_method: "B3LYP",
    orca_basis: "def2-TZVP",
    orca_extra_keywords: "D3BJ",
    charge: 0,
    multiplicity: 1
  }
})

Thermochemistry at non-standard conditions

ORCA computes thermochemistry at 298.15 K and 1 atm by default. For other conditions, the Gibbs energy correction can be computed via the gibbs_energy analysis task:

catgo_workflow_engine(action: "add_task", params: {
  workflow_id: "<wf_id>",
  task_type: "gibbs_energy",
  depends_on: ["opt1", "freq1"],
  params: {
    temperature: 373.15,
    phase: "gas"
  }
})

Get frequency results

catgo_workflow_engine(action: "get_result", params: {
  workflow_id: "<wf_id>",
  task_id: "freq1"
})

The result contains:

• frequencies: list of vibrational frequencies in cm-1
• intensities: IR intensities in km/mol
• is_imaginary: boolean flags for each frequency
• zpe: zero-point energy in eV
• thermochemistry: dict with H, S, G at standard conditions

Interpreting Results

Minima verification

• All frequencies should be real (positive)
• Small negative frequencies (<50 cm-1) are numerical noise, usually harmless
• Large imaginary frequencies indicate the structure is NOT a minimum

Transition state verification

• Exactly ONE imaginary frequency (negative value)
• The imaginary mode should correspond to the expected reaction coordinate
• Use catgo_view to visualize the mode

Thermochemistry output

ORCA prints a thermochemistry block with:

Quantity	Symbol	Units
Zero-point energy	ZPE	eV (or kcal/mol)
Thermal energy	U	eV
Enthalpy	H = U + pV	eV
Entropy	S	eV/K
Gibbs free energy	G = H - TS	eV

For catalysis, feed the DFT energy and frequencies into gibbs_energy:

• phase: "adsorbed" -- harmonic approximation (no translational/rotational)
• phase: "gas" -- ideal gas (includes translation, rotation, vibration)

Frequency Scaling Factors

DFT frequencies are systematically overestimated. Common scaling factors:

Method	Scaling factor
B3LYP/def2-SVP	0.9813
B3LYP/def2-TZVP	0.9654
PBE/def2-SVP	0.9948
HF-3c	0.86

These are applied automatically by the gibbs_energy task when available.

Common Mistakes

• Running freq on unoptimized geometry (will show spurious imaginary modes)
• Using different method/basis for opt and freq (inconsistent PES)
• Ignoring imaginary frequencies and proceeding with thermochemistry
• Not using TightOpt for the preceding optimization (loose opt can leave residual forces that appear as small imaginary frequencies)

ORCA-Specific Notes

• ORCA uses analytical frequencies when available, numerical otherwise
• For large molecules (>100 atoms), frequencies become very expensive
• orca_extra_keywords: "NumFreq" forces numerical frequencies (slower but sometimes needed for exotic functionals)
• ORCA output lists frequencies as negative values for imaginary modes (not "i" notation)