VALBOND force field

Another approach that is conceptually similar is to make certain constants depend on bond order or bond hybridization. Thus, for instance, in the VALBOND force field, angle bending energies at metal atoms are computed from orbital properties of the metal-ligand bonds in the MM2 and MM3 force fields, stretching force constants, equilibrium bond lengths, and two-fold torsional terms depend on computed n bond orders between atoms. 

A very promising recent approach to modeling angular geometries, the VAL-BOND model[30], is based on Pauling s 1931 paper1311 that established the fundamental principles of directed covalent bonds formed by hybridization. The VALBOND force field, which uses conventional terms for bond stretching, torsions, improper tor-... 

Structure optimization of main group molecules with generic force fields has the same advantages and problems as similar calculations of organic and organo-metallic compounds on one hand there is no need to fit a specialist force field, on the other is the expectation of lower accuracy[89,126]. The structural results are especially poor for molecules where electronic effects are important, e.g., those with hypervalent or dative bonds. An exception to this is the VALBOND force field which generally leads to excellent results (see above) [78,81l... 

More recently, the Landis group" has used valence bond concepts to derive new angular potential energy functions and to develop rules for parameterization. The VALBOND force field angular energy function is based on Pauling s hybrid orbital nonorthogonality functions and can be derived for any arbitrary combination of s, p, d, and f orbitals. 

The molecular structures used in the calculations were optimized using a molecular force field program. The force field parameters were derived from various sources The bond length was taken into accound according to a model of Dewar and Llano (7). The VALBOND method of Root et al. (8) was used for the calculation of bond angels. The dihedral angles were parametrized according to the PIMM method (9). 

In Eqs. (2) and (3), % is electronegativity, n is the bond order, and A, is a parameter. Another example of a rule-based force field is VALBOND, from the Landis group. 

Nevertheless, the geometiy-based hybrid orbital strength functions turned out to be useful to capture the angular parts of force fields. Therefore, VALBOND is used in conjunction with a conventional force field such as CHARMM, where the typical harmonic bending terms are replaced with a new parametrization. In fact, VALBOND was developed together with CHARMM [59]. The additional energy term is of the form... 

Refining Force Field Parameters in VALBOND TRANS Application to an Octahedral Iridium-PHOX Catalyst... 

In the following, the VALBOND-TRANS force field is parametrized for model octahedral complexes of Ir, with the aim to best capture relative energies calculated from DPT for different diastereomers. The (re-)parametrization of VALBOND-TRANS is important for successful use in further atomistic simulations. The fitting procedure can be conveniently carried out by using a recent combination of CHARMM and the I-NoLLS fitting environment [79, 80]. This parameter optimization environment can carry out interactive nonlinear least-square fittings to determine model parameters for a wide variety of applications. 

The Force Field As illustrated in Fig. 3, the force field for Pt[Cl2(6-DPPon)2] is decomposed into three parts the metal center Pt and the hgand atoms P and Cl that are treated with VALBOND the double proton transfer (DPT) motifs N-H-N and O-H-0 which are described by MMPT potentials and the remainder of the molecule for which the conventional CHARMM force field is used. 

For the particular case of VALBOND-TRANS, a more comprehensive fitting of an integrated force field is an obvious next step. Until now it was demonstrated that inclusion of electronic effects such as the trans-influence is possible and leads to quantitatively meaningful results when compared with reference DFT calculations. However, for a dedicated force field, all terms (including the bonded ones) need to be fitted. An important question will be how transferable the resulting FF parameters are for the standard chemical atom types (H, C, N, O) between complexes with different transition metals, or whether independent parametrizations are required for different metals and different oxidation states. 

The combination of largely independent but mutually beneficial techniques such as VALBOND-TRANS and MMPT as shown in Sect. 3.2 will open up new possibilities in organometallic chemistry. Such extensions can also be envisaged between VALBOND-TRANS and adiabatic reactive MD (ARMD) [88], which will allow reactions involving TMs to be followed explicitly. A prerequisite for such applications is to more routinely adjust force field parameters for particular chemistries. 

The alternative approach is the generic FF in which some or all the parameters are derived from atom-based properties. The DREIDING FF and the universal force field (UFF) are examples of this approach. They avoid the necessity of developing new parameters every time a new type of bond is encountered but can suffer from reduced accuracy relative to a specialist FF. The VALBOND FF takes elements from both approaches and uses a conventional para-metrization for all terms except the angle bend, where general but fitted atom-pair-based parameters are used. ... 

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