XO: An extended ONIOM method for accurate and efficient geometry optimization of large molecules

A new hybrid scheme, termed as XO, is developed here to surmount the inherent limitations of the widely used ONIOM method. XO features the ability to combine any number of sub-systems described at any level of theories into a compound calculation. We show, as examples, that XO can be applied to zeolites and polypeptides modeling with great satisfaction.