I am reconstructing the force constants from the weighted hessian matrix, as calculated using HF/6-311+G(2d,2p) in Gaussian G09.
The mathematics simple used a submatrix of the atoms of interest (A---B) from the weighted hessian matrix, and projects it onto the bond eigenvectors.
I am running a lot of test cases, one of which is CH4. I would have expected all four bond force constants to be identical. Unfortunately I get:
C1-H2 70 kcal/(mol ang^2)
C1-H3 70 kcal/(mol ang^2)
C1-H4 204 kcal/(mol ang^2)
C1-H5 204 kcal/(mol ang^2)
My maths and my code seems to be correct (from many test e.g., the bond force constant of both O-H in water are BOTH approx 144 kcal/(mol ang^2) at the same level of theory and basis set). I am guessing the results are a direct product of the frequency analysis calculations in Gaussian. Any suggestions?
Thanks
Anthony
May be it is not global, but local minimum. See geometry of your molecule. Try symmetrical configuration and compare with your previous minimum.
This is symmetry sequence. (No Jahn-Teller in ground state!) A number of IR bands are connecdted with two sets of force constants.
Not understandable, Boris. Plenty of Jhan-Teller effects in initially symmetric ground states.
Jahn-Teller needs degeneracy. Ground state of CH4 is not degenerate. Even for pseudo J-T effect the exciateited states are too high in energy.
More symmetric is molecule, more degenerate is it's ground state. What is ground state of CH4?
Turns out my programming and maths are correct, and it is a results of the theory/basis set:
HF/6-311++g(2d,2p): --NOT SYMMETRICAL LOCAL FORCE CONSTANTS
C1-H2 70 kcal/(mol ang^2)
C1-H3 70 kcal/(mol ang^2)
C1-H4 204 kcal/(mol ang^2)
C1-H5 204 kcal/(mol ang^2)
B3LYP/6-311++g(2d,2p): --NOT SYMMETRICAL LOCAL FORCE CONSTANTS
C1-H2 61 kcal/(mol ang^2)
C1-H3 61 kcal/(mol ang^2)
C1-H4 187 kcal/(mol ang^2)
C1-H5 187 kcal/(mol ang^2)
MP2/aug-cc-PVQZ: --SYMMETRICAL LOCAL FORCE CONSTANTS
C1-H2 63 kcal/(mol ang^2)
C1-H3 63 kcal/(mol ang^2)
C1-H4 63 kcal/(mol ang^2)
C1-H5 63 kcal/(mol ang^2)
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