VALBOND

VALBOND Transition metal Atomic-orbital- Root, D. M., Landis, C. R., and Cleveland. T. ... 

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. 

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). 

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-... 

Some of the more approximate or specialized FFs encountered in the literature include CFF, CHEM-X, COSMIC, CYFF, DREIDING, MMX, SHAPES, TRIPOS, VALBOND, and UFF. Well-known FFs for modeling proteins and nucleic acids include AMBER, CHARMM, ECEPP, GROMOS, OPLS and their variants. Some of these latter FFs compromise the quality of reproducing subtle intramolecular electronic effects for the sake of being fast enough to treat biomacromolecules in long molecular simulations. 

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. 

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. 

Valbond . [Air Prods.] Acrylic emulsions print binders, stabilizer fenr textile q)plics. 

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. 

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