Compute gradient, Hessian matrix

[aromanro]

The derivatives could be computed numerically (central difference would be my choice).
In fact, they could be computed 'analytically', but I don't think I would use that method for this project, I suppose the post-HF things would complicate things even more.

They could be used for optimizations computations, that is, computation of molecular structure.
Also vibrational modes & frequencies could be computed.
Ideally, also the lines intensity...