Gaussian 03 Online Manual
Last update: 2 October 2006

Geom

The Geom keyword specifies the source of the molecule specification input. By default, it is read from the input stream, as described previously. Geom may be used to specify an alternate input source. It also controls what geometry-related information is printed and use of internal consistency checks on the Z-matrix. The Geom keyword is not meaningful without at least one item selection option.

ITEM SELECTION OPTIONS

Checkpoint
Causes the molecule specification (including variables) to be taken from the checkpoint file. Only the charge and multiplicity are read from the input stream. For example, Geom=Checkpoint may be used by a later job step to retrieve the geometry optimized during an earlier job step from the checkpoint file. This action is safe since Gaussian will abort the job if an optimization fails, and consequently subsequent job steps which expect to use the optimized geometry will not be executed. May be combined with the ModRedundant option if you want to retrieve and alter the molecule specification in a checkpoint file using redundant internal coordinate-style modifications.

AllCheck
Causes the molecule specification (including variables), the charge and multiplicity, and the title section to be taken from the checkpoint file. Thus, only the route section and any input required by keywords within it need be specified when using this option. This option is not valid with Modify but may be combined with ModRed.

Step=N
Retrieves the structure produced by the Nth step of a failed or partial geometry optimization (it is not valid for a successful optimization). Step=Original recovers the initial starting geometry. This option is used for restarting geometry optimization from intermediate points. It must be combined with one of Checkpoint, AllCheck or Modify. Note that not all steps are always present in the checkpoint file; a Hessian updated message in the log file means that the corresponding step is available in the checkpoint file.

ModRedundant
Modify the current geometry (regardless of its coordinate system) using redundant internal coordinate modifications before performing the calculation. This option may be used to modify a geometry specified in the input file using these features even when some calculation type other than an optimization is to be performed. It may also be combined with Step, Check or AllCheck to retrieve and modify a geometry from a checkpoint file.

The ModLargeRedundant variation uses the minimal setup for Opt=Large. It may not be used for periodic boundary calculations.

When used with Check or Step, two input sections will be read: the first contains the charge and multiplicity, and the second contains alterations to the retrieved geometry. When combined with the AllCheck option, only the geometry modifications input is needed.

Modification specifications for redundant coordinates have the same format as the input for the ModRedundant option of the Opt keyword (we summarize these formats only briefly here; see the discussion of the Opt keyword for a full description):

[Type] N1 [N2 [N3 [N4]]] [[+=]Value] [Action [Params]] [[Min] Max]]

N1, N2, N3 and N4 are atom numbers or wildcards. (numbering begins at 1 and any dummy atoms are not counted.) Value gives a new value for the specified coordinate, and +=Value increments the coordinate by Value.

Action is an optional one-character code letter indicating the coordinate modification to be performed, sometimes followed by additional required parameters (the default action is to add the specified coordinate):

  • B     Add the coordinate and build all related coordinates.

  • K     Remove the coordinate and kill all related coordinates containing this coordinate.

  • A     Activate the coordinate for optimization if it has been frozen.

  • F     Freeze the coordinate in the optimization.

  • R     Remove the coordinate from the definition list (but not the related coordinates).

  • S n stp      Perform a relaxed potential energy surface scan. Set the initial value to Value (or its current value), and increment the coordinate by stp a total of n times, performing an optimization from each resulting starting geometry.

  • H dv     Change the diagonal element for this coord. in the initial Hessian to dv.

  • D     Calculate numerical second derivatives for the row and column of the initial Hessian for this coordinate.

An asterisk (*) in the place of an atom number indicates a wildcard. Min and Max define a range (or maximum value if Min is not given) for coordinate specifications containing wildcards. The Action is taken only if the value of the coordinate is in the range.

Type can be used to designate a specific coordinate type (by default, the coordinate type is determined automatically from the number of atoms specified):

  • X     Cartesian coordinates. In this case, Value, Min and Max are each triples of numbers, specifying the X,Y,Z coordinates.

  • 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 bending coordinate for a center (N1) and three connected atoms.

Modify
Specifies that the geometry is to be taken from the checkpoint file and that modifications will be made to it. A total of two input sections will be read: the first contains the charge and multiplicity, and the second contains alterations to the retrieved geometry. Note that in Gaussian 03, Modi is the shortest valid abbreviation for this keyword.

Modification specifications for geometry optimizations using Z-matrix coordinates have the following form: variable [new-value] [A|F|D]

where variable is the name of a variable in the molecule specification, new-value is an optional new value to be assigned to it, and the final item is a one-letter code indicating whether the variable is to be active (i.e., optimized) or frozen; the code letter D requests numerical differentiation be performed with respect to that variable and activates the variable automatically. If the code letter is omitted, then the variable's status remains the same as it was in the original molecule specification.

Connect
Specify explicit atom bonding data via an additional input section (blank line-terminated) following the geometry specification and any modification to it. This option requires one line of input per atom, ordered the same as in the molecule specification, using the following syntax:

N1 Order1 [N2 Order2 …]

where the N's are atoms to which the current atom is bonded, and the Order's are the bond order of the corresponding bond. For example, this input specifies that the current atom is bonded to atoms 4 and 5, with bond orders of 1.0 and 2.0 respectively:

8 4 1.0 5 2.0 -1.0

This input section is terminated by a blank line.

ModConnect
Modify the connectivity of the atoms in the molecule specification (or retrieved from the checkpoint file). This option requires an additional input section (blank line-terminated) following the geometry specification and any modification to it. Connectivity modifications use the following syntax:

M N1 Order1 [N2 Order2 …]

where M is the atom number, the N's are atoms to which that atom is bonded, and the Order's are the bond order of the corresponding bond. A bond order of -1.0 removes a bond. For example, this input specifies that atom 8 is bonded to atoms 4 and 5, with bond orders of 1.0 and 2.0 respectively, and removes any bond to atom 9:

8 4 1.0 5 2.0 9 -1.0

ZMConnect
Read connectivity using the atom numbering specified in the Z-matrix (including dummy atoms). Bond orders involving dummy atoms are discarded.

IHarmonic=n
Add harmonic constraints to the initial structure with force constant n/1000 Hartree/Bohr2. InitialHarmonic is a synonym for this option.

ChkHarmonic=n
Add harmonic constraints to the initial structure saved on the checkpoint file with force constant n/1000 Hartree/Bohr2. CHarmonic is a synonym for this option.

ReadHarmonic=n

Add harmonic constraints to an additional structure read in the input stream (in the input orientation), with force constant n/1000 Hartree/Bohr2. RHarmonic is a synonym for this option.

OldRedundant
Use the Gaussian 94 redundant internal coordinate generator.

OUTPUT-RELATED OPTIONS

Distance
Requests printing of the atomic distance matrix (which is the default for molecules with fewer than 50 atoms). NoDistance suppresses this output.

Angle
Requests printing of the interatomic angles, using the Z-matrix to determine which atoms are bonded. The default is not to print unless some atoms are specified by Cartesian coordinates or an optimization in redundant internal coordinates is being performed. NoAngle suppresses this output.

CAngle
Requests printing of interatomic angles using distance cutoffs to determine bonded atoms. The default is not to print unless at least one atom is specified using Cartesian coordinates. Only one of Angle, CAngle, and NoAngle may be specified.

Dihedral
Specifies printing of dihedral angles using connectivity information from the Z-matrix to decide which atoms are bonded (the default is not to print). NoDihedral suppresses this output.

CDihedral
Requests printing of dihedral angles using distance cutoffs to determine connectivity. Only one of Dihedral, CDihedral, and NoDihedral may be specified.

PrintInputOrient
Include the table giving the Cartesian coordinates in the input orientation.

GEOMETRY SPECIFICATION AND CHECKING OPTIONS

KeepConstants
KeepConstants retains and NoKeepConstants discards information about frozen variables. The default is to retain them in symbolic form for the Berny algorithm, and to discard them for older optimization algorithms (which don't understand them anyway).

KeepDefinition
Retains the definition of the redundant internal coordinates (the default). Its opposite is NewDefinition.

NewRedundant
Rebuilds the redundant internal coordinates from the current Cartesian coordinates. If used with Geom=Modify, the new modifications are appended to any earlier Opt=ModRedundant input before the coordinate system is updated.

Crowd
Crowd activates and NoCrowd turns off a check which aborts the job if atoms are closer than 0.5 Å. By default, the check is done at the initial point, but not at later points of an optimization.

Independent
Independent activates and NoIndependent turns off a check on the linear independence of the variables specified in a Z-matrix. This is done by default only if a full optimization is requested using the Berny algorithm (Opt=Z-matrix).

MODEL BUILDER OPTIONS

ModelA, ModelB
These options specify that model builder [500] connectivity information will be read and used to construct a symbolic Z-matrix. This option is implemented only for H through Ne, and in some cases will not generate a symbolic Z-matrix with the correct symmetry-constrained number of variables. If geometry optimization has been requested and this problem occurs, the job will be aborted.

Print
Turns on additional printing by the model builder facility.

Guess=Read, Opt=ModRedundant