This provides a computationally efficient quantum-chemical route to the relevant anharmonic energy levels. In addition to a conventional, harmonic frequency calculation (which requires one calculation of the Hessian matrix), only a fixed number of additional singlepoint energy calculations per considered mode is required. This is significantly more efficient than other quantum-chemical methods for the calculation of anharmonic energy levels, such as second-order vibrational perturbation theory, which requires at least two additional calculations of the Hessian per considered normal mode.