Obtaining Vibrational Frequencies and Thermodynamic Properties

To obtain harmonic vibrational frequencies and thermodynamic properties, make sure you have the correct equilibrium geometry for the given level of theory and that the level of theory has been specified. Then simply select QChem Vibrations under the Compute menu. Q-Chem will run in a separate window and will perform a second derivative calculation (either an analytic second derivative for Hartree-Fock, or a numerical second derivative by differentiating first derivatives for DFT and MP2). This will provide all the information needed for harmonic vibrational frequencies and thermodynamic properties, which will be printed out towards the end of the output. As a check, for an analytic second derivative computation, your total energy should be exactly the same as at the end of the optimization. For a numerical second derivative calculation, a series of gradients will be evaluated, and the energy at each point should be within 0.001 hartree of the equilibrium total energy found at the end of optimization. If not, you have the wrong geometry or the program is finding the wrong solution.