On the degeneracy of the Hessian matrix

Abstract

A widespread notion in the computational chemistry literature about the Hessian matrix has been revisited, namely, that the Hessian matrix over Cartesian space is sixfold degenerate due to the three translational and three rotational degrees of freedom. It has been shown that this is true only at critical points on the potential energy hypersurface, otherwise the Hessian matrix is only threefold degenerate. The rotational degrees of freedom generally do not cause degeneracy in the Hessian matrix away from critical points.