The change of sign makes the value of the force consistent with
the definition of the force ( -grad(E) )
A positive value of the constraint force tends to increase the
constraint value.
The change of sign of PairForce makes it such that a positive
PairForce value tends to increase the bond length.
The value of the PairForce needed to compensate for a distance
constraint is opposite to the force acting on the constraint.

Add calculation of eigenvalues and eigenfunctions.
Modify command syntax to include optional energy range,
optional gaussian width.
Changed parameter de to const value (0.01).