Initiates the definition of a symmetry operation referred to by label (any three characters).
sign can be + or 
; it specifies whether the total wavefunction is symmetric or
antisymmetric under this operation, respectively. A value for sign is not always necessary
but, if provided, constraints will be put on the structure coefficients to ensure that the
wavefunction has the correct overall symmetry (note that the configuration list must be closed
under the orbital permutation induced by label for this to be possible).
The operator is defined in terms of its action on the active MOs as specified by one or more of
the keywords IRREPS, COEFFS, or TRANS (any other keyword will terminate the
definition of this symmetry operator). If no further keyword is supplied, the identity
is assumed for label. The alternative format
SYMELM,label,sign={key-1,...;key-2,...;...} may also be used.