Molecular Symmetry

PSI3 can determine automatically the largest Abelian point group for a valid framework of centers (including ghost atoms, but dummy atoms are ignored). It will then use the symmetry properties of the system in computing the energy, forces, and other properties. However, in certain instances it is desirable to use less than the full symmetry of the molecule. The keyword subgroup is used to specify a subgroup of the full molecular point group. The allowed values are c2v, c2h, d2, c2, cs, ci, and c1. For certain combinations of a group and its subgroup there is no unique way to determine which subgroup is implied. For example, $D_{\rm 2h}$ has 3 non-equivalent $C_{\rm 2v}$ subgroups, e.g. $C_{\rm 2v}(X)$ consists of symmetry operations $\hat{E}$, $\hat{C}_2(x)$, $\hat{\sigma}_{xy}$, and $\hat{\sigma_{xz}}$. To specify such subgroups precisely one has to use the keyword unique_axis. For example, the following input will specify the $C_{\rm 2v}(X)$ subgroup of $D_{\rm 2h}$ to be the computational point group:

  psi: (
    ...
    geometry = (
         ...
    )
    units = angstrom
    subgroup = c2v
    unique_axis = x
  )

Point Group	Cotton Ordering of Irreps
C$_1$	A
C$_i$	A$_g$ A$_u$
C$_2$	A B
C$_s$	A' A''
C$_{2h}$	A$_g$ B$_g$ A$_u$ B$_u$
C$_{2v}$	A$_1$ A$_2$ B$_1$ B$_2$
D$_2$	A B$_1$ B$_2$ B$_3$
D$_{2h}$	A$_g$ B$_{1g}$ B$_{2g}$ B$_{3g}$ A$_u$ B$_{1u}$ B$_{2u}$ B$_{3u}$