Frozen core orbitals as an alternative to specific frontier bond potential in hybrid Quantum Mechanics/Molecular Mechanics methods

The use of Frozen Core Orbitals (FCOs) at the frontier atom is proposed as an alternative to the specific frontier bond potential in the Local-SCF/MM method. The calculations illustrate that frontier bond length is correctly recovered without adding any specific force-field potential. It is shown that the same accuracy is reached for calculations using FCOs arising either from an atomic (ROHF) or from a molecular calculation using Extremely Localized Molecular Orbitals (ELMOs). In addition FCOs do not modify the description of the remaining of the QM subsystem from both an energetic and/or geometric point of view.