Function compute_vibrational_analysis 

pub fn compute_vibrational_analysis(
    elements: &[u8],
    positions: &[[f64; 3]],
    method: &str,
    step_size: Option<f64>,
) -> Result<VibrationalAnalysis, String>

Perform vibrational analysis via numerical Hessian.

Computes vibrational frequencies (cm⁻¹), normal modes, IR intensities, and zero-point vibrational energy from a semiempirical Hessian.

method: “eht”, “pm3”, or “xtb”