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16.6 Using additional point-group symmetry

Since *MOLPRO* can handle only Abelian point-groups, there may be more
symmetry than explicitly used. For instance, if linear molecules are treated
in instead of , the
-orbitals
appear in symmetry 1 (). In other cases, a linear geometry may occur as a
special case of calculations in symmetry, and then one component of the
-orbitals occurs in symmetry 1 (). The program is able to detect such
hidden “extra” symmetries by blockings in the one-electron hamiltonian and
the overlap matrix . Within each irreducible representation, an “extra”
symmetry number is then assigned to each basis function. These numbers are
printed at the end of the integral output. Usually, the extra symmetries are
ordered with increasing -quantum number of the basis functions. This
information can be used to determine and fix the extra symmetries of the
molecular orbitals by means of the `SYM` command.
`SYM`,

are the extra symmetries for the first orbitals in the
irreducible representation *irrep*. For instance, if you want that in a linear
molecule the orbitals 1.1 to 3.1 are and 4.1, 5.1 , the `SYM`
card would read (calculation done with X,Y as symmetry generators):

`SYM,1,1,1,1,2,2`

If necessary, the program will reorder the orbitals in each iteration to force
this occupation. The symmetries of occupied and virtual orbitals may be
specified. By default, symmetry contaminations are not removed. If *irrep* is
set negative, however, symmetry contaminations are removed. Note that this may
prevent convergence if degenerate orbitals are present.

molpro@molpro.net 2019-06-17