How to get rid of imaginary frequencies within ONIOM geometry optimizations: A DFT study on the effect of basis set and link atom distances in Cu-ZSM-5

Two extended clusters representing different portions of Cu-ZSM-5 were treated within a two-layer ONIOM approximation, employing DFT calculations for both the real and the model system. Despite a two-step optimization procedure successfully employed in previous work, a consistent number of imaginary and anomalous frequencies appeared after the vibrational analysis. These artefacts depend both on the basis set assigned to link atoms and on an improper setting of the O–H distances, where H are the link atoms at the boundaries of the model system. The latter problem, revealed for the first time in the present study, originates from the default scale factor employed by the ONIOM routine within Gaussian-09. Once basis set and g scale factor are properly set, all imaginary and anomalous frequencies disappear. The present findings may represent an interesting and practical solution to an annoying computational problem, whenever it occurs in the framework of ONIOM calculations.