Constraining Coordinates in Jaguar Calculations

When you optimize the geometry of a molecule, you might want to freeze certain coordinates, or constrain them to be equal to each other. Freezing coordinates reduces the number of free parameters in the optimization, and may reduce the number of steps to convergence, or allow you to converge a difficult optimization in stages. Constraining coordinates can be used to enforce symmetry, in either Z-matrix coordinates or Cartesian coordinates.

To constrain specific coordinates to their original values during an optimization, you can use the controls in the Add New Constraint section of the Optimization tab, or you can edit the input file.

Dynamic constraints, also called “soft” or “harmonic” constraints, are implemented by means of Lagrange multipliers. A dynamic constraint on a geometric coordinate is met gradually during the course of an optimization. One advantage of using a dynamic constraint on a variable is that you can choose a value that is different from its current value. For example, if you have a structure whose conformation you want to change, and you know that changing a particular torsional angle would cause parts of the molecule to collide if the torsional angle’s value were suddenly imposed, you can instead specify the desired value for the torsion as a dynamic constraint. The optimizer changes the torsion gradually during the optimization, so that the final torsional angle is as close as possible to the desired torsional angle.

Defining dynamic constraints is handled in the coord section, which is described in The coord and connect Sections of the Jaguar Input File. You can make a constraint dynamic by editing the input file or by using the controls in the Optimization tab.