GeomThe
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 geometryrelated
information is printed and use of internal consistency checks on the Zmatrix.
The Geom keyword is not meaningful without at least one item selection
option. ITEM SELECTION OPTIONSCheckpoint 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
coordinatestyle 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 N^{th} 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 onecharacter 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 Outofplane
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 Zmatrix coordinates have the following form: variable [newvalue]
[AFD] where variable is the name of a variable in the molecule
specification, newvalue is an optional new value to be assigned to it,
and the final item is a oneletter 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 lineterminated)
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
lineterminated) 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 Zmatrix (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/Bohr^{2}.
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/Bohr^{2}. 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/Bohr^{2}. RHarmonic
is a synonym for this option. OldRedundant Use the Gaussian
94 redundant internal coordinate generator. OUTPUTRELATED OPTIONSDistance
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 Zmatrix 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 Zmatrix 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 OPTIONSKeepConstants 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 Zmatrix. This is done by default only if a full
optimization is requested using the Berny algorithm (Opt=Zmatrix). MODEL
BUILDER OPTIONSModelA, ModelB These options specify that model
builder [500] connectivity information will be
read and used to construct a symbolic Zmatrix. This option is implemented only
for H through Ne, and in some cases will not generate a symbolic Zmatrix with
the correct symmetryconstrained 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 