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10.2 Symmetry specification

If standard Z-matrix input is used, MOLPRO determines the symmetry automatically by default. However, sometimes it is necessary to use a lower symmetry or a different orientation than obtained by the default, and this can be achieved by explicit specification of the symmetry elements to be used, as described below.

Generating symmetry elements, which uniquely specify the point group, can be specified on the SYMMETRY directive. This must be given before the geometry block. Each symmetry directive only affects the subsequent geometry block; after a geometry block has been processed, the defaults are restored. Note that the specification of symmetry elements inside the geometry block is no longer allowed.

The dimension of the point group is 2**(number of fields given). Each field consists of one or more of X, Y, or Z (with no intervening spaces) which specify which coordinate axes change sign under the corresponding generating symmetry operation. It is usually wise to choose $z$ to be the unique axis where appropriate (essential for $C_2$ and $C_{2h}$). In that case, the possibilities are:

(null card)
$C_1$ (i.e., no point group symmetry)
Z
$C_s$
XY
$C_2$
XYZ
$C_i$
X,Y
$C_{2v}$
XY,Z
$C_{2h}$
XZ,YZ
$D_2$
X,Y,Z
$D_{2h}$
Note that Abelian point group symmetry only is available, so for molecules with degenerate symmetry, an Abelian subgroup must be used -- e.g, $C_{2v}$ or $D_{2h}$ for linear molecules.

See section 4.8 for more details of symmetry groups and ordering of the irreducible representations. Also see section 10.1.1 for more information about automatic generation of symmetry planes.

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.



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