# Most efficient level of theory for sulfur-containing proline analogues (one that will work)

I'm modelling a small set of proline analogues - extra functional groups, sulfur in the ring. Using Gaussian / DMACRYS.

--functional B3LYP EmpiricalDispersion=GD3BJ --basis_set 6-311G**


or

-fun B3LYP EmpiricalDispersion=GD3BJ -bas 6-311G**

1. Would EmpiricalDispersion=GD3BJ go under -fun, or would it go under --additional_args? (If so, please show how, I can't get this to run correctly, unsure of the syntax)

2. To model this more accurately, PBEPBE has been suggested, pure DFT rather than hybrid functional, so there is increased accuracy. Is there a sufficient increase in accuracy for the increase in computational time, or is it better to apply dispersion corrections to B3LYP? (Or PBEPBE and corrections?) Is there another level of theory you would suggest trying?

3. The electrostatic interactions seem to be over-modelled using B3LYP / 6-311G**, is there a basis set / functional combination (without or with corrections) that would not over-estimate the energy of the + / - charge interactions? (I've tried CAM-B3LYP, but doesn't work on my system.)

• It would significantly help if you could indicate how you're running Gaussian. My guess is that the empirical dispersion correction goes under "additional_args" but you seem to be using some program to create the Gaussian input, so it depends on that. – Geoff Hutchison Apr 20 '15 at 12:40
• @GeoffHutchison is quite right, it would be interesting to have the Gaussian input file. Pure DFT would cut down a little on the computational cost, if it enhances accuracy is probably something only benchmarks can tell you. – Martin - マーチン Apr 20 '15 at 12:53