I'm trying to set up the simulation portion of the classic undergraduate lab studying the rotavibrational spectrum of HCl and DCl. In terms of simulation (using Gaussian 16), the equilibrium geometry and vibrational energy calculations are trivial. Including anharmonicity is also simple through the use of the freq=anharm
keyword. I'm running into trouble with the inclusion of vibrational-rotational parameters, which the program is supposed to support, but I can't find appropriate output information. I'm either missing the output data because I don't know what I'm looking for, or something is wrong in the calculation. Specifically, I'd like the students to get ab initio values for $D_e$, $\widetilde{\nu}_0$, $B_e$, and $\alpha_e$ in the following equation ($m$ is a substitution for $-J''$ or $J''+1$ for the $P$ and $R$ branches, respectively):
$$\widetilde{\nu}(m) = \widetilde{\nu}_0 + (2 B_e - 2 \alpha_e) m - \alpha_e m^2 - 4 D_e m^3$$
I'm currently able to find $D_e$ and $\widetilde{\nu}_0$.
The only clue I can find in the calculation is the following lines:
WARNING: Anharmonic treatment of linear tops is experimental.
Moreover, an hybrid treatment is used to simulate spectra:
- Energy: equations including degenerate modes are used.
- Intensity: summation done on N' modes, considering only one mode
per couple of degenerate modes. No variational correction done.
==================================================
Coriolis Couplings
==================================================
[No Coriolis Coupling for diatomic molecules
Later, in the entry for Vibro-Rot alpha Matrix (where I'd expect to see entries for $\alpha_e$), I get:
Vibro-Rot alpha Matrix (in cm^-1)
---------------------------------
A(z) B(x) C(y)
Q( 1) -0.00000 NaN NaN
I'm assuming the message for Coriolis Couplings is connected to the zero-valued alpha matrix. Is there a way to pull out these parameters in a Gaussian simulation? If not, is there another ab initio package that will perform these calculations? I've tried (without luck) to look through the NWChem
documentation.
The input might be useful to reproduce the calculation. Unfortunately, the output is larger than SE.com will allow. Let me know if you require additional lines from it (or a full copy).
Input file:
%nproc=4
%mem=400MB
# MP2 def2tzvp
# opt freq=(anharm,vibrot,noraman) scf=tight freq=ReadIsotopes
Vibrational analysis for D35Cl
0 1
Cl 0.0 0.0 0.0
H 0.0 0.0 1.27
298.15 1.0
35
2
Update:
After more reading through the Gaussian references, I've found the rotational constants (Ba(x)
, Ca(y)
) under the following section:
==================================================
Vibrational Energies at Anharmonic Level
==================================================
Units: Vibrational energies and rotational constants in cm^-1.
NOTE: Transition energies are given with respect to the ground state.
Reference Data
--------------
E(harm) E(anharm) Ba(x) Ca(y)
Equilibrium Geometry 10.643785 10.643785
Ground State 1525.714 1515.023 NaN NaN
Fundamental Bands
-----------------
Mode(n) Status E(harm) E(anharm) Ba(x) Ca(y)
1(1) active 3051.429 2951.452 NaN NaN
Overtones
---------
Mode(n) E(harm) E(anharm) Ba(x) Ca(y)
1(2) 6102.858 5802.928 NaN NaN
WARNING: Anharmonic transition moments for symmetric and linear tops
are not yet fully implemented.
However, I still can't find an entry for $\alpha_e$, the coupling constant between vibrational and rotational states. Is Gaussian capable of putting out this constant? If so, am I looking for it under the wrong name?
#P
to get more output, there might be more stuff hidden between the defaults. Use checkpoint files (%Chk
). Split the calculation into one optimisation, and a frequency calculation (you can use%OldChk
). I have made the experience that specifying keywords twice may cause problems for g16, so combine the option stacks. I try look into this deeper, but i can't promise $\endgroup$#P
doesn't seem to give me any additional relevant information. I did go ahead and remove thefreq=ReadIsotopes
and just specified directly in the atom specificationsH(Iso=2)
. $\endgroup$