I am working with acetone molecule and have done frequency analysis(both harmonic and an-harmonic). I want to fit c=o stretching mode to Morse potential. Is there any way to get the Morse potential parameters?
Also can we get cubic and quartic force constants in Gaussian or gamess?
Normal mode cannot provide you the morse potential. You must scan the potential energy surface and then get the curve, with which you can fit a morse potential.
Zhaoxi Sun Do you kow how to fit it to a morse potential? If you have any reference paper or something it will be helpful.
Doing an anharmonic vibrational analysis In Gaussian will give you the cubic and quartic force constant matrices ( semi-diagonal ). They are used for perturbative anharmonic corrections to the harmonic vibrational frequencies.
I have done some basic calculation previously. Just scan the pes and fit a function with least square fitting. You need to write a code yourself for the scanning and fitting.
Use relax scan along the specific mode and fit the values in morse potential to get the parameters.
For fitting u can use gnuplot in linux based systems @lokhande
Expand the Morse potential in a Taylor series at r = re (minimum of the potential) and truncate the series at the quadratic term. Equate this to the harmonic potential expression: (1/2).k.x^2 = (1/2).[2.De.(a^2)].x^2 where De = classical dissociation energy, a = Morse exponential parameter, and x = r - re. In other words, the harmonic force constant k = [2De.(a^2)]. You can calculate k from the harmonic normal mode frequency. Now, you have to supply the De value either from experiment or from computation, and you can calculate the exponential parameter. To be consistent, De can be calculated as the difference between the sum of the energies of C and O (ground states) and the CO molcule using the same method and basis set as the molecule (perhaps with Poisson correction), or from a potential scan as suggested by Frederick Bennet (this is reliable only if you are using a multi-reference method). Be careful with units as you do all this. The attached PDF contains a (very old) MathCAD exercise in which I show how to obtain the Morse potential curve for the N2 molecule based on the experimental data for atomization energy, harmonic frequency, and equilibrium bond distance.
Alternately, as Frederick suggested, you can do an anharmonic frequency analysis and get the anharmonic correction, expand the Taylor series expansion of the Morse potential to higher order terms, and then with a bit of effort, the Morse exponential parameter can be extracted.
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