Is there a way to get a vibrational frequency for a given molecule?
  Yes, there are several ways to determine the vibrational frequencies of a molecule. Here are some common methods:

1. Infrared (IR) spectroscopy: IR spectroscopy is a common technique used to measure the vibrational frequencies of molecules. In this technique, a molecule is illuminated with IR radiation, and the absorbed energy is measured as a function of wavelength. The vibrational frequencies of the molecule can be determined by analyzing the absorption spectrum.
2. Raman spectroscopy: Raman spectroscopy is another technique used to measure the vibrational frequencies of molecules. In this technique, a molecule is illuminated with a laser of a specific wavelength, and the scattered light is measured as a function of wavelength. The vibrational frequencies of the molecule can be determined by analyzing the Raman spectrum.
3. Nuclear magnetic resonance (NMR) spectroscopy: NMR spectroscopy is a technique used to measure the vibrational frequencies of atoms in a molecule. In this technique, a magnetic field is applied to the molecule, and the nuclear spins of the atoms in the molecule are excited. The vibrational frequencies of the molecule can be determined by analyzing the NMR spectrum.
4
In quantum chemistry, vibrational frequencies of a molecule can be calculated using various methods. Here are a few ways to obtain vibrational frequencies for a given molecule:

1. **Harmonic Frequency Calculation**: This is a common method used in quantum chemistry software packages like Gaussian, ORCA, or Psi4. The calculation involves computing the Hessian matrix (a matrix of second derivatives of the energy with respect to nuclear coordinates) and then diagonalizing it to obtain the vibrational frequencies.
2. **Density Functional Theory (DFT)**: DFT is a popular method for calculating vibrational frequencies. It involves solving the Kohn-Sham equations to obtain the electronic density, and then using the resulting density to compute the vibrational frequencies.
3. **Molecular Mechanics (MM)**: MM is a classical mechanics-based method that uses force fields to describe the interactions between atoms in a molecule. Vibrational frequencies can be calculated by diagonalizing the mass-weighted Hessian matrix.
4. **Semi-Empirical Methods**: Semi-empirical methods, such as PM3 or AM1, use a simplified Hamiltonian and parameterized functions to describe the electronic structure of a molecule. Vibrational frequencies can be calculated using these methods, although the accuracy might be lower compared to more advanced methods.
5. **Experimental Methods**: Vibrational frequencies can also be measured experimentally using techniques like Infrared (IR) spectroscopy, Raman spectroscopy, or Neutron Inelastic Scattering (NIS