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Dear all!

I've got a monoclinic molecular crystal with Br, O, C and H. In order to relax the structure without any modification of the unit cell which criterium do you consider to be more adquate, reducing the energy or decreasing the stress (forces) on atoms? Which values would be more appropiate in either case?.The final goal is to calculate the vibrational spectrum on the optimized geometry

Thanks

first optimize the structure via stress/force optimization (ISIF=3). Use high precision and increase the E-cutoff to do so. Furthermore, restart from the CONTCAR of a pre-optimized structure (after about 5 ionic steps) without using the WAVECAR (especially if V- and shape change significantly in the first steps) to make sure that there are no basis-set related errors.
To check the quality of the converged results, do 2-3 few E/V
steps for that converged geometry.