AIDER allows MOPAC to optimize an ab initio geometry. To use
it, calculate the ab initio gradients using, e.g.,
Gaussian [14]. Supply MOPAC with these gradients, after
converting them into kcal/mol. The geometry resulting from a MOPAC run
will be nearer to the optimized ab initio geometry than if a single step
were taken using the geometry optimizer in Gaussian.