Angle-bending force constant

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. 

In these equations, m is the mass of an element (a CHa group), kc is the force constant for stretching of a C—C bond, ka is the force constant for bending of the CCC angle, and

phase difference between the motions of adjacent elements of the chain. Since we are interested only in the factor group modes, i.e those in which the vibrations of corresponding elements in neighboring unit cells are in phase, we require that... 

I lic 3-alorn and 4-atoni rcstrairiis behave just like those for two atoms, except tliat the dialog hox requests an angle rather than a distance and the accompanying request for a force constant requires either a bending force constant (3-atorn case) or a torsional force con Stan t (4-atorn case). 

When the same energy terms are used in two force helds, it may be acceptable to transfer bond-stretching and angle-bending terms. These are fairly stiff motions that do not change excessively. The force constants for these terms vary between force helds, much more than the unstrained lengths and angles. 

A larger value for the bending force constant K0 leads to a greater tendency for the angle to remain at its equilibrium value 0g. There may be cubic, quartic, etc. terms as with the corresponding bond stretch term in addition to the quadratic term shown here. 

The functional form for angle bending in BIO+ is quadratic only and is identical with that shown in equation (12) on page 175. The angle bending force constants are in units of kcal/mol per radian2 and are in the file pointed to by the QuadraticBend entry for the parameter set in the Registry or the chem.ini file, usually called ben.txt(dbf). 

Ground-state effects, i.e., the effects on the bond lengths and angles, stretching and bending force constants, and other spectroscopic properties of the other ligands. 

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