Energy gradients are easy to compute, in the restricted SCF
model, provide an insight into reaction mechanisms, and make
possible the efficient computation of force constants, equilibrium
geometries, and the structures of transition states.3- 6 However, there
are some serious hazards in the routine use of the gradient programs
now becoming widespread, associated with the instability of symmetry-
restricted SCF solutions to symmetry-breaking. Lowering of
the electronic energy associated with the symmetry breaking· in the
wave functions may be induced deliberately by choice of the basis
set, or unintentionally by the nuclear distortions which are introduced
in the course of calculations of force constants. If the latter,
· an artificially low, and perhaps negative force constant will be
computed, associated with a »normal coordinate« which breaks the
molecular symmetry. This will produce false conclusions on the ease
of symmetry breaking distortions. We show by reference to a model
two-electron system that this problem is frequently encountered in
twixtyl or diradical systems, and can be overcome by choosing a
wavefunction which is not symmetry or spinrestricted, without the
need for extensive configuration mixing
