To investigate whether finite-difference calculations of frequencies (using analytic gradients) can be carried out in single precision, we computed the frequencies and normal modes for benzene at the respective optimized geometries with the same convergence criteria as used for geometry optimizations with the step size of 0.001 Å. The resulting frequencies are very different: real and imaginary frequencies of ∼15000 cm -1 occur along with several imaginary frequencies. Thus, not surprisingly, single precision is not sufficient for finite-difference calculations. How-ever, finite-difference calculations using single-precision calculation with the double-precision cleanup step fully recovers double precision frequencies, while affording ∼ ×1.3 speedup of the calculation.