Self-Consistent Strictly Localized Orbitals.

Among all the Quantum Mechanics/Molecular Mechanics (QM/MM) methods available to describe large molecular systems, the Local Self-Consistent Field/MM (LSCF/MM) one uses frozen doubly occupied Strictly Localized Bonding Orbital (SLBO) to connect the QM fragment to the one treated at the MM level. This approach is correct as long as the QM part is large enough to minimize the artifacts that could arise because of the fixed SLBO. If one wants to decrease the size of the QM subsystem, one clearly needs to help the SLBO to relax according to the variations of the global wave function. Also, the SLBO have to adjust itself according to the modification of the surrounding if we want to improve the method. Here, we present a modification of the original LSCF method called Optimized LSCF (OLSCF) where each SLBO is allowed to mix with its corresponding Strictly Localized Anti Bonding Orbital (SLABO) resulting in an adjustment of the two-electron bond described by a self-consistent SLBO (SCSLBO). We test the new methodology against the modification of the QM part (internal perturbation) and against the variation of the surroundings (external perturbation) represented either by a dielectric continuum or by a classical point charge. In each case the initial SLBO is the symmetric C-C SLBO of the ethane molecule. It is shown that the optimized SCSLBO presents a final polarity in perfect agreement with what could be expected as the result of a reaction to the internal or external perturbation.