In this exercise, you will determine the optimum geometry for gaseous hypochlorous acid, HOCl. Namely, you will determine the two bond lengths and one bond angle that minimize the potential energy due to all nuclear and electronic interactions. Furthermore, you will determine the vibrational frequencies of HOCl using a normal mode analysis, which treats the potential surface in the vicinity of the minimum as being harmonic and calculates the corresponding 3N - 6 (for non-linear molecule) or 3N - 5 (for a linear molecule) vibrational modes, each corresponding to a concerted motion of all atoms. For a non-linear triatomic, this corresponds to a symmetric and antisymmetric stretch and a bending motion. For a linear triatomic, this corresponds to symmetric and antisymmetric stretch and two degenerate bending motions. Once you have calculated the geometry and vibrational frequencies, they can be compared with experimental measurements.
While computational chemistry can be a powerful tool, the quality of results depends on the accuracy of the electronic structure method employed and the atomic orbital basis used. In this activity, you can compare geometries and vibrational frequencies calculated using Hartree-Fock, Density Functional Theory, or any method supported by the QuantumChemistry package. Note that geometry optimizations for more sophisticated methods can be computationally expensive!