Subject: isotope vibrational frequency calculations

From: "Gavin Tsai" <[email-masked]>
Date: Fri, 1 Oct 1999 15:03:31 -0500

Hi,

    This is a question at the vibration and isotope vibration calculations
in G98W.
The simple HCl molecule is used as an example to describe my question.
Below lists the results of HCl and deturated DCl HF/6-31G** vibration
calculations:

HCl                          DCl
1                            1
SG                           SG
Frequencies -- 3179.3881     Frequencies -- 2280.2669
Red. masses -- 1.0360        Red. masses -- 2.1231
Frc consts -- 6.1702         Frc consts -- 6.5041
IR Inten -- 34.0816          IR Inten -- 17.5309
Raman Activ -- 116.6559      Raman Activ -- 60.0055
Depolar -- 0.3772            Depolar -- 0.3772

By comparing those calculation, it is very reasonable to observe the
frequency of DCl is lower than HCl and the reduced masses of DCl is
larger than that in HCl.
The question is the force constants are different in those calculations.
Theoretically, the force constants are not mass dependent. Are they
mass-weighted? How does G98w calculate the reduced masses and force
constants in the vibration calculations?

Thanks!

Gavin Tsai