I was trying to do raman calculation from a peptide which has 320 atoms using gaussian calculation. I have used 3-21g basis , DFT method and B3LYP functional. I used avogadro to optimized it before runing in guassin. I am getting convergence error. Could anybody help me figure out the problem ? Thanks
Dear Dr. Khanra,
can you please specify what kind of convergence error you got? Does your structure not converge after maximum number of optimization cycles at all?
Best regards,
Aleksey.
You can do the following: calculate frequencies for your structure, then, if there are any imaginary frequencies, you should take the XYZ-coordinates (under "Standard orientation" in the output) of your structure from the frequency calculation, put them into the input and make some changes to them. Also, you should use the command line like this:
#B3LYP/3-21G scfcyc=1000 guess=read opt=(readfc,maxcycle=500)
Generally, it should help. Also, I would advise to use the 3-21G* basis set rather than 3-21G, it would better describe chemical bonding. Btw, do you run your calculations in the gas phase or in the solvent?
Best,
Aleksey.
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