Restrained optimizations

Up to now, the only way to constrain distances, angles or dihedral angles within a geometry optimization is by using a Z-matrix, and freezing that particular coordinate. With the key RESTRAINT it is possible to select any coordinate (distance, angle, dihedral), irrespective of the coordinates used, and restrain this coordinate.

Note the difference between constrained and restrained coordinates. At every step in the geometry optimization, the value of a constrained coordinate should match exactly a predefined fixed value. On the other hand, with restraints, a potential is added to the potential energy in order to satisfy the restraint, which means that the restraint does not have to be satisfied exactly. For example, one can start with a geometry in a geometry optimization run in which the restraint is not satisfied.

The RESTRAINT keyword allows geometry optimizations with restraints for the distance between two atoms, an angle defined by three atoms, (and) or a dihedral angle defined by four atoms:

RESTRAINT
 DIST Ia1 Ia2 Ra {[Aa] [Ba]}
 ANGLE Ib1 Ib2 Ib3 Rb {[Ab] [Bb]}
 DIHED Ic1 Ic2 Ic3 Ic4 Rc {[Ac] [Bc]}
end

DIST

When DIST is specified, the distance between atoms Ia1 and Ia2 is restrained to the value Ra. The atom numbers should be given in Input order; the value for the distance in Angstrom. The Aa and Ba values are mere technical values, that don't have to be specified (in fact, recommended not to change these values); the default values of 2.0 resp. 0.1 have been chosen on sensible grounds.

ANGLE

When ANGLE is specified, the angle between atoms Ib1, Ib2 and Ib3 (Ib1-Ib2-Ib3) is restrained to the value Rb. The atom numbers should be given in Input order; the value for the angle in degrees. The Aa and Ba values are mere technical values, that don't have to be specified (in fact, recommended not to change these values); the default values of 1.0 resp. 0.1 have been chosen on sensible grounds.

DIHED

When DIHED is specified, the dihedral angle between atoms Ic1, Ic2, Ic3 and Ic4 (Ic1-Ic2-Ic3-Ic4) is restrained to the value Rc. The atom numbers should be given in Input order; the value for the angle in degrees. The Aa and Ba values are mere technical values, that don't have to be specified (in fact, recommended not to change these values); the default values of 0.5 resp. 0.1 have been chosen on sensible grounds. The dihedral angle Ic1-Ic2-Ic3-Ic4 is defined in the same way as for the Z-matrix in ADF. The dihedral angle is projected onto the [0,2π] interval, so there should be no difference between specifying -30° or 330°.