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talbers Offline OP
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I would like to find good values for two dihedrals in methacrylophenone: the one from the ipso-carbon to the methyl group (orange in the attached .png) and the one from the carbonyl oxygen to the methyl group (red in the .png).

Since these two dihedrals are always 180 deg out of phase I could adjust one or the other to fit the potential from QM calculations. What would be the most suitable way to set these values? Should I fit the CG331-CG2DC1-CG2O5-OG2D3 in, say, 3-Methyl-3-buten-2-one first and adjust the CG331-CG2DC1-CG2O5-CG2R61 torsion to fit that, or am I overthinking the problem?

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Last edited by talbers; 11/18/14 05:41 AM.
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Force field design is relatively detailed work, and people usually tend to underthink more than overthink. Your question is a pertinent one. First of all, for the record, only one potential energy scan is needed per rotatable bond. This leaves you to fit 3 redundant dihedral parameters to 1 energy profile. What I usually do in this kind of situation is constrain the parameters to be synergetic and have the same absolute amplitude, as this tends to decrease the chance of unwanted forces and improve transferability. Since your phase difference is 180°, this means that the even multiplicities should get the same phase and the odd ones opposite phase. Your case will likely at least need a 1-fold and a 2-fold, so your fitting problem would look like this:
Code:
CG2DC3 CG2DC1 CG2O5  CG2R61     x       1     0.00
CG2DC3 CG2DC1 CG2O5  CG2R61     y       2   180.00
CG331  CG2DC1 CG2O5  CG2R61     x       1   180.00
CG331  CG2DC1 CG2O5  CG2R61     y       2   180.00
CG331  CG2DC1 CG2O5  OG2D3      x       1     0.00
CG331  CG2DC1 CG2O5  OG2D3      y       2   180.00
Where x and y are the sought-after variables. In the above formalism, negative solutions are acceptable, but by convention, we make them positive again by flipping the corresponding phases in the final parameter file. In case you need further explanation with all this, this would be a good starting point.

Last edited by Kenno; 11/18/14 04:42 PM. Reason: added 2 sentences: "First of all ... to 1 energy profile."

Moderated by  alex, lennart, rmv 

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