Frequency Analysis Examples

Frequency analysis computes harmonic vibrational frequencies, IR intensities, and thermochemical properties from the nuclear Hessian. OpenQuantum supports two paths:

• Analytical Hessian (default for RHF/UHF/RKS/UKS) — uses the fully analytical energy second derivative with CPHF response
• Semi-analytical Hessian — finite-difference of analytical gradients

The thermochemistry module adds zero-point energy, thermal corrections (U, H, G), and entropy via the RRHO model with automatic symmetry-number detection.

See the sub-pages for full examples of each approach.