Symmetry

The Overlap and Fock matrices become block-diagonal by using symmetry-adapted combination of the (C)BAS functions, such that each such function transforms under the symmetry operators as one of the subspecies of the irreducible representations (irrep) of the symmetry group. Symmetry adapted functions are denoted (C)SBAS.

For a given irrep and subspecies not all elementary basis functions can participate in the symmetry adapted combinations. For instance, for an atom in a reflection plane a basis function that is antisymmetric with respect to the reflection cannot be part of any symmetric combination of functions. In particular for higher symmetries the number of BAS functions that are relevant for a subspecies may be considerably smaller than the total number of BAS functions. This is used to cut down expansion lengths, both as used internally in the computation and construction of the Fock matrix, and in printed output. The printed expansion coefficients (in the bas representation) refer only to the participating BAS functions. A defining list of them is printed at an early stage of the run for each of the subspecies.