I try to parameterize the triple C-=C bond. I used the model CH3-CH2-C-=C-H. I believe I have made the UMAT correct, as the vibrational frequencies from guessed+exist Charmm force field are tidy (only two negative eigenvalues; 1-3 modes contribute to one frequency; no negative contribution; reasonable frequencies for known vibrational modes). However, when I tried to use the same UMAT to calculate the frequencies from Gaussian results, it is a mess (too many negative eigenvalues; many negative contributions to some frequencies; the frequencies for the known modes (exist bonded parameters) are not correct). I tried using different levels of theory and/or different initail conformations in Gaussian calculations, but no luck.

In g03, I used

#MP2/6-31G* OPT POP=(MK,DIPOLE) DENSITY=CURRENT

for optimization and

#MP2/6-31G* GEOM=ALLCHECK FREQ IOP(7/33=1)

or

#MP2/6-31G* GEOM=ALLCHECK Opt=CalcAll IOP(7/33=1)

for freqency calculation.

Any suggests are highly appreciated.

Shunzhou

btw, months ago, Tina asked about C-=C bond parameters, any luck for him?