The matrix of second partial derivatives of a function (assumed to be twice differentiable). This is often denoted H_f(x), where f is the function and x is a point in its domain.
The matrix containing all of the second partial derivatives of the cost function, , with respect to the control variables. Element of the Hessian is (see Eq. (8)).
Matrix of mass normalized 2nd derivatives of energy with respect to nuclear displacement in Cartesian coordinates. Diagonalized to determine normal modes & calculate vibrational frequencies.