MP2 is not used for obtaining the electrostatic potential to fit. However, as an aside, MP2 is used for deriving conformational energies when fitting torsional parameters.
To use RESP, you must first generate an electrostatic potential (or charge grid, colloquially). This is discussed a bit in the Appendices. MOPAC does both grid generation and the charge fit, so in effect hides the process. RESP just does the fit, so a program is required to generate the grid for RESP to fit on. To derive charges according to the method used in the Cornell et al. force field, the grid should be generated using a full quantum treatment (instead of semiempirical per MOPAC), i.e. using a program such as g94/g98 or GAMESS, optimizing at the 6-31G* level of basis set (or possibly 6-31G** if the molecule is very electronegative). Assuming that you have studied the parameter generation and charge-fitting sections of the Appendix plus the force field papers, the next step is to choose which quantum program to use, obtain it, and use it to optimize your molecule at the 6-31G* level, perhaps using multiple conformations if these are important. Then you will calculate/export the grid in that program for use with RESP.
handy script/program to convert g94 output Note that this script ONLY works for single point Gaussian runs, NOT geometry optimizations!!
%chk=gaussfile.chk # Geom=coord # HF/6-31G* # Prop=(Potential,Read) water to use RESP 0 1 8 0.000000 0.000000 0.231385 1 0.000000 1.494187 -0.925538 1 0.000000 -1.494187 -0.925538 -9.9821230e-33 0.0000000e+00 3.9352480e+00 1.8519310e+00 0.0000000e+00 3.4390240e+00 9.2596570e-01 1.6038200e+00 3.4390240e+00 -9.2596570e-01 1.6038200e+00 3.4390240e+00 ...........295 points altogether..........
The mistake is using angstroms in the .dat file instead of bohrs.
Note - HF is used for the potential here (as opposed to optimization).
Because the nitro and trifluoromethyl groups are polar, they should be equivalenced at the first stage. Doing so at the second stage would cause a marked deterioration in the quality of the fit around the important polar areas of these groups.
Second stage equivalencing is only for atoms describing non-polar areas of the electrostatic potential.
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