Specifying Coordinates for Hessian Refinement

If you are optimizing a molecular structure to obtain a transition state, you might want to refine the Hessian used for the job. Transition-State Optimizations explains the methods used for transition-state optimizations, including Hessian refinement. This topic explains how to edit your input file to specify particular coordinates for Hessian refinement. (Whether or not you refine particular coordinates, you can specify a certain number of the lowest eigenvectors of the Hessian for refinemen—the Hessian can be refined in both ways in the same job.)

If you type an asterisk (*) after a coordinate value, Jaguar computes the gradient of the energy both at the original geometry and at a geometry for which the asterisk-marked coordinate has been changed slightly, and uses the results to refine the initial Hessian to be used for the optimization. To request refinement of a coordinate whose value is set using a variable, add an asterisk to the end of the variable setting in the variable definition section.

For instance, either of the following two input geometries will have the same effect: a job that includes Hessian refinement will use both O–H bonds and the H–O–H angle in the refinement.

O1
H2   O1   1.1*
H3   O1   1.1*   H2   108.0*

or

O1
H2   O1   ohbond
H3   O1   ohbond   H2   108.0*
ohbond = 1.1*

Molecular symmetry or the use of variables, either of which may constrain several coordinate values to be equal to each other, can reduce the number of coordinates actually used for refinement. For example, for the second water example shown above, only two coordinates are actually refined (the O–H bond distance, which is the same for both bonds, and the H–O–H angle). The same would be true for the first example if molecular symmetry were used for the job.