The Opt=ModRedundant keyword adds, deletes or modifies redundant internal coordinate definitions (including scan and constraint information). This option requires a separate input section following the geometry specification. When used in conjunction with QST2 or QST3, a ModRedundant input section must follow each geometry specification. AddRedundant is synonymous with ModRedundant. The input for Geom=ModRedundant is the same.
Lines in a ModRedundant input section use the following syntax:
[Type] N1 [N2 [N3 [N4]]] [[+=]value] [A | F] [[min] max]]
[Type] N1 [N2 [N3 [N4]]] [[+=]value] B [[min] max]]
[Type] N1 [N2 [N3 [N4]]] K | R [[min] max]]
[Type] N1 [N2 [N3 [N4]]] [[+=]value] D [[min] max]]
[Type] N1 [N2 [N3 [N4]]] [[+=]value] H diag-elem [[min] max]]
[Type] N1 [N2 [N3 [N4]]] [[+=]value] S nsteps stepsize [[min] max]]
N1, N2, N3 and N4 are atom numbers or wildcards (discussed below). Atom numbering begins at 1, and any dummy atoms are not counted. Value specifies a new value for the specified coordinate, and +=value increments the coordinate by value.
The atom numbers and coordinate value are followed by a one-character code letter indicating the coordinate modification to be performed; the action code is sometimes followed by additional required parameters as indicated above. If no action code is included, the default action is to add the specified coordinate. The available action codes are listed below, and the corresponding GaussView Redundant Coordinate Editor bottom popup menu item is indicated by bold type (the menu's Add menu item does not add an action code):
| Code | Meaning |
| none | Add the specified coordinate (Add). |
| A | Activate the coordinate for optimization if it has been frozen. |
| F | Freeze the coordinate in the optimization. |
| B | Add the coordinate and Build all related coordinates. |
| K | Remove the coordinate and Kill all related coordinates containing this coordinate. |
| R | Remove the coordinate from the definition list (but not the related coordinates). |
| D | Calculate numerical second Derivatives for the row and column of the initial Hessian for this coordinate. |
| H | Change the diagonal element for this coordinate in the initial Hessian to diag-elem. |
| S | Perform a relaxed potential energy surface Scan. Set the initial value of this coordinate to value (or its current value), and increment the coordinate by stepsize a total of nsteps times, performing an optimization from each resulting starting geometry. |
An asterisk (*) in the place of an atom number indicates a wildcard. Min and max then define a range (or maximum value if min is not given) for coordinate specifications containing wildcards. The action specified by the action code is taken only if the value of the coordinate is in the range.
Here are some examples of wildcard use:
* All atoms specified by Cartesian coordinates
* * All defined bonds
3 * All defined bonds with atom 3
* * * All defined valence angles
* 4 * All defined valence angles around atom 4
* * * * All defined dihedral angles
* 3 4 * All defined dihedral angles around the bond connecting atoms 3 and 4
When the action codes K and B are used with one or two atoms, the meaning of a wildcard is extended to include all applicable atoms, not just those involving defined coordinates.
By default, the coordinate type is determined from the number of atoms specified: Cartesian coordinates for 1 atom, bond stretch for 2 atoms, valence angle for 3 atoms and dihedral angle for 4 atoms. Optionally, Type can be used to designate these and additional coordinate types (the corresponding GaussView Redundant Coordinate Editor upper popup menu item is indicated by bold type):
| Code | Meaning |
| X | Cartesian Coordinate. In this case, value, min and max are interpreted as the X, Y and Z coordinates (respectively). |
| B | Bond length. |
| A | Valence Angle. |
| D | Dihedral angle. |
| L | Linear Bend specified by three atoms (or if N4 is -1) or by four atoms, where the fourth atom is used to determine the 2 orthogonal directions of the linear bend. In this case, value, min and max are each pairs of numbers, specifying the two orthogonal bending components. |
| O | Out-of-Plane Bend for a center (N1) and three connected atoms. |
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