Optimizing a Geometry

Once you have drawn your molecule, obtained a reasonable guess geometry using the Model Build option, and specified the level of theory, you can tell Q-Chem to find the equilibrium geometry at that level of theory by going to the Compute menu and specifying QChem Geometry Optimization. Q-Chem will run in a separate window and produce quite a lot of output. Finding the optimum geometry is an iterative process, requiring an energy and gradient computation at a series of geometries. If your initial guess for the geometry is poor or if there is some other problem, the geometry optimization may not converge! Look carefully towards the end of your output to see if Q-Chem says it has converged or not. Once you have converged on the optimum geometry (and perhaps even before then?), the picture of your molecule in HyperChem will be updated with the new geometry. The total energy, orbital energies, and other properties for the equilibrium geometry may be found in the output towards the end, in the final geometry cycle. A common mistake is to take data from previous geometry points before the geometry has been converged. Only the final geometry point corresponds to the optimized geometry.