Generation of the FORCE matrix is very time-consuming, and in isotopic substitution studies several vibrational calculations may be needed. To allow the frequencies to be calculated from the (constant) force matrix, `ISOTOPE` is used. When a `FORCE` calculation is completed, `ISOTOPE` will cause the force matrix to be stored, regardless of whether or not any intervening restarts have been made. To re-calculate the frequencies, etc., starting at the end of the force matrix calculation, specify `
RESTART`.

The two keywords `RESTART` and `ISOTOPE` can be used together. For example, if a normal `FORCE` calculation runs for a long time, the user may want to divide it up into stages and save the final force matrix. Once `ISOTOPE` has been used, it does not need to be used on subsequent `RESTART` runs. `ISOTOPE` can also be used with `FORCE` to set up a `RESTART` file for an `
IRC= n` calculation.