The Default MM Force Field

Add any of the currently named 1-, 2-, 3-, or 4-atom selections to the list of restraints. 

The 3-atom and 4-atom restraints behave just like those for two atoms, except that the dialog box requests an angle rather than a distance and the accompanying request for a force constant requires either a bending force constant (3-atom case) or a torsional force constant (4-atom case). 

These molecular dynamics restraints are stored with the HIN file and are retained as long as the Named Selections are still active (structural changes not made to molecular system) or the restraints are still requested via the Restraint Forces dialog box. 

The default restraints are appropriate for molecular dynamics calculations where larger force constants would create undesirable high frequency motions but much larger force constants may be desired for restrained geometry optimization. 

One of the major difficulties with molecular mechanics procedures (MMh- or otherwise) is that they almost always fail. That is, you find that force constants are not available for the molecule of interest. This is both the strength and weakness of molecular mechanics it uses atom types to introduce specific chemical environments for the atoms within a molecule (to obtain accuracy in the calculations) but then requires knowledge of force constants specific to that chemical environment (as specific as stating that an atom is in a five-member ring containing one oxygen and one carbon, for example). As the number, N, of atom types rises the number of force constants needed to describe all possible occurrences of these atom type becomes very large. For torsions, for 

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