Using Geometric Restraints

You can include geometric restraints—for interatomic distances, bond angles, and torsion angles—in any molecular dynamics calculation or geometry optimization. Here are some applications of restraints  

Restraints add potential terms to a force field calculation, favoring the value that you specify in a restraint. The larger the value of the h arm on ic force con stan t, th e m ore tigh tly th e calculation restrain s the value. 

For all restraints, HyperChem uses named selections that contain two, three, or four atoms each. You use Name Selection on the Select menu to assign names to groups of selected atoms. Then you can apply named selections as restraints for a calculation in the Restraint Forces dialog box from Restraints on the Setup menu. 

Caution Restraints may significantly affect the property you are observing. When using restraints, check their effects on measured values. 

---

Note You can superimpose harmonic restraining forces to interatomic distances, angles, or dihedrals that you have set up as named selections. Yon can also restrain atoms to points in space. See Using Geometric Restraints" on page SI and "Restraints" on page 105. 

See Using Geometric Restraints on page 81 for examples ofusing restraints. 

Conformations of the radicals were then computed ab initio. This required spin-unrestricted calculations to be performed, which when combined with the system size made for quite demanding calculations at this time. The computed values of geometrical parameters could then be used as restraints in the least-squares refinement process, as outlined in Section 3.9, and in this way the structures of the P[CH(SiMe3)2]2 and As[CH(SiMe3)2]2 radicals were determined [91, 92]. 

In a first approximation, RR is related to Bricogne s likelihood estimation [46]. Although RR is a reciprocal-space measure, it is obtained from computer simulation in real space this allows the inclusion of arbitrarily complex restraints such as geometric energy functions. The likelihood estimation is formulated in reciprocal space, which makes it prohibitively difficult to include useful real-space restraints. 

---