Sloppy modes

Many molecules have sloppy modes, implying that geometric departures along these modes from the true minimum hardly change the energy and do not result in sizeable gradients. This usually shows up in slow convergence: energy and gradients appear to be converged but the computed step lengths, an assessment of the error in the geometry itself, do not disappear.

Starting from ADF2005.01 delocalized coordinates can be used in geometry optimizations and transition state searches. The use of delocalized coordinates often help in convergence of these problematic sloppy modes.

It depends then on the purpose of the run whether a continued search for the minimum is useful if one has slow convergence: not if you only want to know the energy at the minimum, but certainly so if you want to determine all geometric parameters to high precision. Depending on the case you may therefore want to relax the convergence criterion on the coordinate steps. In the case of Z-matrix optimization this has to be done primarily for the angular coordinates because the bond lengths are usually much stiffer and will therefore not suffer from sloppy mode problems. If you insist on strict convergence of sloppy modes you should use a fair integration precision (at least 4.0, preferably 5.0).