geometry={ 4 FORMALDEHYDE C 0.0000000000 0.0000000000 -0.5265526741 O 0.0000000000 0.0000000000 0.6555124750 H 0.0000000000 -0.9325664988 -1.1133424527 H 0.0000000000 0.9325664988 -1.1133424527 }

As seen in the example, the geometry is specified within the *geometry block*,
enclosed by `geometry={` and `}`.
The first line
of the *xyz* geometry block holds the number of atoms (free format).
The second line is an arbitrary title. Finally there is one line
for each atom specifying the cartesian coordinates () in
Ångstrøm. For simplicity, the first two lines can also be omitted.

Alternatively, the geometry can be specified in the
*Z-matrix* form, which is also used in many other programs.
In this
case, the geometry is defined by distances and angles. This is a
little more involved, and here we give only
a simple example, again for formaldehyde.

angstrom geometry={ C O C 1.182 H1 C 1.102 O 122.1789 H2 C 1.102 O 122.1789 H1 180 }

Here, 1.182 and 1.102 are the C-O and the C-H bond distances, respectively.
122.1789 is the H-O-C angle, and 180 is the angle between the H1-C-O and
H2-C-O planes (dihedral angle). Note that by default the bond distances
are in atomic units (bohr), but one can give the `angstrom` keyword to
use Ångstrøm.

As an alternative to the *xyz* input explained above, it is also
possible to specify cartesian coordinates in a Z-matrix. In this case the form
is

angstrom geometry={ C,, 0.0000000000 , 0.0000000000 , -0.5265526741 O,, 0.0000000000 , 0.0000000000 , 0.6555124750 H,, 0.0000000000 , -0.9325664988 , -1.1133424527 H,, 0.0000000000 , 0.9325664988 , -1.1133424527 }

Again, atomic units are the default, other than
for the *xyz* input, where the coordinates need to be given
in Ångstrøm. In order to use Ångstrøm, the ` angstrom` command
must be given before the geometry block.

Instead of constant numbers it is also possible to use variables in the Z-matrix input. For instance, the input for formaldehyde can be written as

geometry={ C O , C , rco H1 , C , rch , O , hco H2 , C , rch , O , hco , H1 , 180 } rco=1.182 Ang rch=1.102 Ang hco=122.1789 Degree

The values of the variables can be changed in the course of the calculations, so that calculations can be performed for different geometries in one run. This will be explained in more detail later.

By default, the program repositions and rotates the molecule so that
the coordinate axes have the centre of mass of the molecule as origin,
and are aligned with the principal inertial axes. This behaviour is
usually what is required, in order that the program can recognize and
exploit point-group symmetry, but can be changed through parameters
specified on the `ORIENT` command (see manual).
By default, the program tries to find and use the maximum abelian point-group
symmetry, but this behaviour can be controlled using the
`SYMMETRY` command (see manual).

molpro@molpro.net 2018-12-16