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The PNO–MP2 gradient and its application to molecular geometry optimisations

ABSTRACT A preliminary implementation of a gradient for MP2 based on the pair natural orbital approximation (PNO–MP2) is presented. The accuracy of the PNO approximation for geometries is investigated by comparing bond lengths and bond angles of the PNO–MP2 structures with results from canonical MP2. Special emphasis is put on the optimisation of weakly bound complexes, such as differently sized water clusters. It is found that the error introduced by the PNO approximation is already for moderate PNO thresholds negligible compared to the inherent error of MP2. However, too loose PNO thresholds lead to convergence difficulties in geometry optimisations. This is observed in particular for floppy groups as, for example, methyl groups with low rotational barriers. Compared to optimisations with canonical MP2, the convergence thresholds have to be mainly energy and less coordinate based to comply with the slight roughness of the potential energy surface which is introduced by the PNO selection.