By default Q-Chem calculates vibrational frequencies using the atomic masses of the most abundant isotopes (taken from the Handbook of Chemistry and Physics, Edition). Masses of other isotopes can be specified using the $isotopes section and by setting the ISOTOPES $rem variable to TRUE. The format of the $isotopes section is as follows:
$isotopes number_of_isotope_loops tp_flag number_of_atoms [temp pressure] (loop 1) atom_number1 mass1 atom_number2 mass2 ... number_of_atoms [temp pressure] (loop 2) atom_number1 mass1 atom_number2 mass2 ... $end
Note: Only the atoms whose masses are to be changed from the default values need to be specified. After each loop all masses are reset to the default values. Atoms are numbered according to the order in the $molecule section.
An initial loop using the default masses is always performed first. Subsequent loops use the user-specified atomic masses. Only those atoms whose masses are to be changed need to be included in the list, all other atoms will adopt the default masses. The output gives a full frequency analysis for each loop. Note that the calculation of vibrational frequencies in the additional loops only involves a rescaling of the computed Hessian, and therefore takes little additional computational time.
The first line of the $isotopes section specifies the number of substitution loops and also whether the temperature and pressure should be modified. The tp_flag setting should be set to 0 if the default temperature and pressure are to be used (298.18 K and 1 atm respectively), or else to 1 if they are to be altered. Note that the temperatures should be specified in Kelvin and pressures in atmospheres.
$molecule 0 1 C 1.08900 0.00000 0.00000 C -1.08900 0.00000 0.00000 H 2.08900 0.00000 0.00000 H -2.08900 0.00000 0.00000 $end $rem BASIS 3-21G JOBTYPE opt METHOD hf $end @@@ $molecule read $end $rem BASIS 3-21G JOBTYPE freq METHOD hf SCF_GUESS read ISOTOPES 1 $end $isotopes 2 0 ! two loops, both at std temp and pressure 4 1 13.00336 ! All atoms are given non-default masses 2 13.00336 3 2.01410 4 2.01410 2 3 2.01410 ! H’s replaced with D’s 4 2.01410 $end