Parse Fermi resonances and IR intensities from ORCA VPT2 output files.

python3 parse_vpt2.py [OPTIONS] file.out

List all Fermi resonances:

python3 parse_vpt2.py data/VPT2_furan_vpt2.out

Filter by Type I only:

python3 parse_vpt2.py data/VPT2_furan_vpt2.out --type 1

Require all coupling modes to have IR intensity ≥ 10 km/mol:

python3 parse_vpt2.py data/VPT2_furan_vpt2.out -I 10

Print the harmonic IR table instead of Fermi resonances:

python3 parse_vpt2.py data/VPT2_furan_vpt2.out --ir-table

• file , required: ORCA VPT2 output file (.out)
• --type {1,2} , optional: filter by resonance type
• -I , --min-intensity N , optional: minimum IR intensity (km/mol) for coupling modes
• --ir-table , optional: print the IR intensity table instead of Fermi resonances

• Type I: ω_i ≈ 2×ω_j — fundamental i is near-degenerate with overtone 2×ω_j
• Type II: ω_i ≈ ω_j + ω_k — fundamental i is near-degenerate with combination ω_j + ω_k

The |Δ| value is the energy difference in cm⁻¹ (denominator). Lower values indicate stronger coupling.

• The number at the end is the denominator = |ω_i - 2ω_j| (Type I) or |ω_i - ω_j - ω_k| (Type II) in cm⁻¹. The threshold 10 means only resonances with denominator < 10 cm⁻¹ are printed. A smaller denominator means a stronger resonance / more severe VPT2 singularity.

• Mode numbers follow ORCA's output conventions (i.e., including translational/rotational modes). The offset (typically 6) is automatically detected from the IR table and applied to all Fermi mode indices.

Contributed by Emmanuel Bourret