SIBFA approach

As the SIBFA approach relies on the use of distributed multipoles and on approximation derived form localized MOs, it is possible to generalize the philosophy to a direct use of electron density. That way, the Gaussian electrostatic model (GEM) [2, 14-16] relies on ab initio-derived fragment electron densities to compute the components of the total interaction energy. It offers the possibility of a continuous electrostatic model going from distributed multipoles to densities and allows a direct inclusion of short-range quantum effects such as overlap and penetration effects in the molecular mechanics energies. 

As electric fields and potential of molecules can be generated upon distributed p, the second order energies schemes of the SIBFA approach can be directly fueled by the density fitted coefficients. To conclude, an important asset of the GEM approach is the possibility of generating a general framework to perform Periodic Boundary Conditions (PBC) simulations. Indeed, such process can be used for second generation APMM such as SIBFA since PBC methodology has been shown to be a key issue in polarizable molecular dynamics with the efficient PBC implementation [60] of the multipole based AMOEBA force field [61]. 

As we have seen, Anisotropic Polarizable Molecular Mechanics (APMM) procedures such as SIBFA or GEM are more complex than usual classical approaches. 

On the basis of a sound analysis of intermolecular interactions, performed by means of a quantum perturbational approach, Claverie derived a force field that could suitably represent intermolecular interactions. The electrostatic interactions are described by means of a distributed multipole analysis, and induction effects are taken into account. The force field sum of interactions between fragments completed ab initio (SIBFA) originated from this study and was subsequently applied successfully to many biophysical problems. 

Figure 8.3 displays four curves, namely the reference ab initio CSOV polarization contribution, the undamped full GEM polarization energy, the full GEM + damping approach, and results obtained upon computing the polarization energy obtained with the exact ab initio undamped field values extracted from a quantum mechanical Gaussian 09 computation. The damping procedure is identical to the one used by SIBFA and is detailed in the technical appendix in (Chaudret et al., 2014). 

Abstract This chapter discusses molecular mechanics (MM)-based approaches to investigate organometallic complexes. In particular, ligand field MM (LFMM), Sum of Interactions Between Fragments Ab Initio (SIBFA), and VALBOND with its extension to VALBOND-TRANS are presented in some detail. Two particular applications of VALBOND-TRANS to an Ir(III) and a Pt(II) complex are presented. Possible future extensions, including the study of chemical reactions and polarization effects, are briefly discussed at the end. 

Ligand Field Ejfects In order to extend the range of application of the SIBFA force field to metals with partially filled d-shells such as Cu(II) with clear biological importance, a LFSE based on the same AOM introduced in the LFMM approach was recently added [38]. The total interaction energy of a metal-Ugand complex (including a dispersion term F isp for improved agreement with correlated ab initio methods) [46] in SIBFA-LF is defined, therefore, as... 

The theoretical investigations included molecular dynamics, SIBFA (Sum of Interactions Between Fragments initio computed), molecular mechanics, HF and DFT calculations (on models of inhibitor-enzyme complexes on small model complexes including 88 atoms, extracted from the 104-residue complexes [53]. Calculations were carried out both with unconelated (HF) as well as correlated (DFT, MP2) quantum chemical approaches. 

A Gaussian-based electrostatic model (GEM) has been explored as an alternative to distributed point multipole electrostatic representation. GEM computes the molecular interaction energies using an approach similar to SIBFA... 

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