Atomic electrostatic parameters scheme

The scheme of electrostatic atomic parameters discussed in Section 4 is still not specific enough to determine the parameters that are necessary for the empirical force field. However, as equations (10a, b) suggest, the source for these parameters should be the molecular multipoles and their derivatives (the AMTs). In Sections 5.1 and 5.2 we show how it is possible to extract the required electrostatic parameters from the AMT in the planar case. 

The principle aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M = Fe(H), Ru(II), Os(II), V(U), Cr(II), Cofll), Co(ni), Fe(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Fig. 13.3) to be accurately reproduced 981. The preferred conformation and the calculated barrier for cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 13.1). This is not surprising since the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters which are dependent on the metal center). An important conclusion was that the preference for an eclipsed conformation of metallocenes is the result of electronic effects. Van der Waals and electrostatic terms were similar for the eclipsed and staggered conformation and the van der Waals interactions were attractive 981. It is important to note, however, that these conclusions are to some extent dependent on the parameterization scheme, and particularly on the parameters used for the nonbonded interactions. 

Partitioning a single bond into two parts involves dividing an electron pair, usually with the QM boundary atom left in an open-shell configuration. Various schemes have been developed to tackle this problem. A straightforward approach is to cap the unsaturated atom with a dummy or so-called link atom, usually a hydrogen. To avoid steric and electrostatic conflicts with the MM subsystem, some bonded parameters and partial charges of nearby MM atoms must be modified. 

The MM parameters at the boundary are used similarly as they are implemented in AMBER for the link atom approach. Internal coordinate force field terms of bonds, angles, and dihedral angles are used if any MM or MMH atom is involved. Electrostatic and van der Waals (vdW) interactions are also calculated in accordance with the AMBER force field scheme [28]. Detailed description of the retained bonded and nonbonded terms, in particular the interactions with bond charges, is discussed in the following section. 

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