10.1.1 Z-matrix input

`[`*group [,]]atom*, , , , , , ,

or, alternatively,

`[`*group [,]]atom*, , , ,

where

*group*- atomic group number (optional). Can be used if different basis sets are used for different atoms of the same kind. The basis set is then referred to by this group number and not by the atomic symbol.
*atom*- chemical symbol of the new atom placed at position . This may optionally be appended (without blank)
by an integer, which can act as sequence number, e.g.,
`C1`,`H2`, etc. Dummy centres with no charge and basis functions are denoted either`Q`or`X`, optionally appended by a number, e.g,`Q1`; note that the first atom in the z-matrix must not be called`X`, since this may be confused with a symmetry specification (use`Q`instead). - atom to which the present atom is connected. This may be either a number
*n*, where refers to the 'th line of the Z-matrix, or an alphanumeric string as specified in the*atom*field of a previous card, e.g.,`C1`,`H2`etc. The latter form works only if the atoms are numbered in a unique way. - Distance of new atom from . This value is given in bohr, unless
`ANG`has been specified directly before or after the symmetry specification. - A second atom needed to define the angle . The same rules hold for the specification as for .
- Internuclear angle . This angle is given in degrees and must be in the range .
- A third atom needed to define the dihedral angle . Only applies if , see below.
- Dihedral angle in degree. This angle is defined as the angle between the planes defined by and ( ). Only applies if , see below.
- If this is specified and nonzero, the new position is specified by two bond angles rather than a bond angle and a dihedral angle. If , is the angle . If , the triple vector product is positive, while this quantity is negative if .
*x,y,z*- Cartesian coordinates of the new atom. This form is assumed if ; if , the coordinates are frozen in geometry optimizations.

All atoms, including those related by symmetry transformations,
should be specified in the Z-matrix.
Note that for the first atom, no coordinates need be given,
for the second atom only are needed, whilst for the
third atom may be omitted.
The 6 missing coordinates are obtained automatically by the
program, which translates and re-orients the molecule such that
the origin is at the centre of mass, and the axes correspond to
the eigenvectors of the inertia tensor (see also `CHARGE` option
above).

Variable names, and in general expressions that are linear in all dependent variables, may be used as well as fixed numerical values for the parameters , and . These expressions are evaluated as late as possible, so that it is possible, for example, to set up loops in which these parameters are changed; the geometry optimizer also understands this construction, and will optimize the energy with respect to the value of the variables. Non-linear expressions should not be used, because the geometry optimization module is unable to differentiate them.

Once the reorientation has been done, the program then looks for
symmetry ( and subgroups), unless the `NOSYM` option
has been given. It is possible to request that reduced symmetry
be used by using appropriate combinations of the
options `X,Y,Z,XY,XZ,YZ,XYZ`. These specify symmetry operations,
the symbol defining which coordinate axes change sign under the
operation. The point group is constructed by taking all combinations
of specified elements.
If symmetry is explicitly specified in this way, the program
checks to see that the group requested can be used, swapping the
coordinate axes if necessary.
This provides a mechanism for ensuring that the same point group
is used, for example, at all points
in the complete generation of a potential
energy surface, allowing the safe re-utilization of neighbouring
geometry molecular orbitals as starting guesses, etc..

Note that symmetry is not implemented in density fitting methods, and in these cases the NOSYM option is implied automatically.

Also note that by default the automatic orientation of the molecule only takes place
if the geometry is defined by internal (Z-matrix) coordinates. In case of `XYZ`
Input the orientation is unchanged, unless the `MASS` option is specified
in the geometry block.

molpro@molpro.net 2018-09-24