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.