By default Q-Chem calculates vibrational frequencies using the atomic masses of the most abundant isotopes. 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 (298.15 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. See Example 10.7.2.
ISOTOPES
ISOTOPES
Specifies if non-default masses are to be used in the frequency calculation.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Use default masses only.
TRUE
Read isotope masses from $isotopes section.
RECOMMENDATION:
None
Example 10.29 An EDF1/6-31G* optimization, followed by a vibrational analysis including isotopic substitution. Doing the vibrational analysis at a stationary point is necessary for the results to be valid.
$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 6-31G*
JOBTYPE opt
METHOD edf1
$end
@@@
$molecule
read
$end
$rem
BASIS 6-31G*
JOBTYPE freq
METHOD edf1
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 ! Hs replaced with Ds
4 2.01410
$end
Example 10.30 EDF1/6-31G* optimization followed by vibrational analysis in which the vibrational enthalpy and entropy and computed at different temperatures and pressures.
$comment Demonstrate use of $isotopes section First, optimize the geometry $end $rem jobtype¯¯opt method¯¯edf1 basis¯¯6-31G* $end $molecule 0 1 O 0.000000 0.000000 0.106731 H -0.758095 0.000000 -0.528927 H 0.758095 0.000000 -0.528927 $end @@@ $comment Next, calculate frequencies for various isotopologues $end $rem jobtype¯¯freq method¯¯edf1 basis¯¯6-31G* isotopes¯true $end $molecule READ $end $isotopes 3 1 ! "1" here means we will set T & P 0 500.0 1.0 ! 500 K, 1 atm, standard masses 0 298.0 10.0 ! 298 K, 10 atm, standard masses 2 500.0 1.0 ! 500 K, 1 atm, subst. D for H 2 2.0141018 3 2.0141018 $end