VALBOND model

Another promising approach to modeling angular geometries, the VALBOND model [115, 116, 122], is based on Paulings 1931 paper [118] that established the fundamental principles of directed covalent bonds formed by hybridization. The... 

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). 

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. 

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. 

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