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

► In XO, both overlapping and onion-like structures can be used. ► In XO, models can be treated at different or same level of theory at wish. ► XO is efficient in alleviating the ONIOM boundary errors. ► XO is robust, being less sensitive to the choice of the low level. ► XO provides more accurate geometries in a more effective way.