The vibrational frequencies computed in the harmonic vibrational analysis
by Gaussian do not only depend on the computed force constants, but
also on the atomic masses. Substitution of, for example, one hydrogen by
a deuterium atom therefore leads to dramatic changes in the calculated
vibrational frequencies. As the Hessian is not affected by the changes in
the masses, the same Hessian can be used for all isotopomers. In order
to read the Hessian out of the checkpoint file and compute the vibrational
frequency spectrum for different masses, the keywords
freq=(ReadFC,ReadIsotopes)
must be combined with additional information of the atomic masses of the
system. Using hydrogen chloride H-Cl as an example, the calculated frequency
for the H-Cl stretching vibration amounts to 3186.1 cm-1 at the
HF/6-31G(d) level of theory and an equilibrium bond length of 126.62 pm.
The vibrational frequency calculation for D-Cl can be executed efficiently
provided that the checkpoint file already contains the Hessian matrix for
H-Cl:
#P HF/6-31G(d) freq=(Readfc,ReadIsotopes)
geom=check guess=read
HF/6-31G(d) freq D-Cl
0 1
298.15 1.0 1.0
34.96885
2
The additional input section after charge and multiplicity information
contains a first line specifying the temperature (in K), the pressure (in atm), and a
uniform scaling factor for all frequencies (normally 1.0).
On subsequent lines the atomic masses are given in the input order, one mass
per line. The atomic masses can either be entered accurately (as a real number)
or as an integer closest to the true mass. Specifying the mass of deuterium using an
integer (2) Gaussian will take the exact mass of the deuterium isotope
(2.01410au) from an internally stored list of values. The vibrational frequency
calculated for D-Cl at the HF/6-31G(d) level of theory using the same geometrical
structure and Hessian as for H-Cl amounts to 2285.04 cm-1.
last changes: 03.02.2005, HZ questions & comments to: zipse@cup.uni-muenchen.de