mat-raman-spectra

Raman Spectra Calculation

Goal

To calculate the Raman spectrum of a crystalline material by:

1. Identifying Raman-active phonon modes via group-theory symmetry analysis of Γ-point phonons (MLIP-only path)
2. Optionally computing full Raman scattering intensities using DFT-level Born effective charges and dielectric tensors (DFT path)

Physical background: Raman scattering intensity of mode $\nu$ is proportional to $|(\hat{e}i \cdot \alpha\nu \cdot \hat{e}s)|^2$, where $\alpha\nu = d\boldsymbol{\alpha}/dQ_\nu$ is the Raman tensor (derivative of polarizability with respect to mode coordinate $Q_\nu$). Computing $\alpha_\nu$ requires DFT-level dielectric calculations; phonon frequencies alone can be obtained with MLIPs.

[!IMPORTANT] Two-tier approach:

• MLIP tier (Step 1–3): Predicts Raman peak positions (frequencies). Intensities are set equal as a placeholder. Sufficient for peak assignment and comparison with experiment when known structure is available.
• DFT tier (Step 4): Computes actual Raman intensities via Born charges and dielectric tensor from VASP DFPT. Required for quantitative intensity matching.

Prerequisites

• Pymatgen, phonopy installed in base-agent (for symmetry analysis and plotting)
• MLIP environment (mace-agent, matgl-agent, or fairchem-agent) for phonon calculation

Instructions

1. Relax Structure

Before computing phonons, ensure the structure is fully relaxed. Use the MCP tool for your chosen MLIP:

# Env: mace-agent
mcp_mace_load_model(model_name="MACE-MH-1")
mcp_mace_relax_structure(
    structure_data="input_structure.cif",
    relax_cell=True,
    fmax=0.001,       # tight convergence for phonons
    output_dir="relaxation/"
)

[!IMPORTANT] Use a tight force convergence (fmax ≤ 0.001 eV/Å) for phonon calculations. Poorly relaxed structures produce imaginary frequencies that indicate a false structural instability.

2. Calculate Phonons (MLIP)

Use the mat-phonon skill to compute Γ-point phonons. The output phonon.yaml is the required input for this skill.