I'm attempting to use Q-Chem 5.0 to replicate CHESHIRE-esque http://cheshirenmr.info/ ab initio calculations of chemical shift values. Unfortunately Gaussian seems to be the predominant tool in this space, and translating from Gaussian16 to Q-Chem is quite a challenge with respect to the solvent model. Currently, just to debug, I am

  1. Performing geometry optimization in Gaussian B3LYP/6-31+G(d,p) (gas phase)
  2. Using those geometries in both Gaussian mPW1PW91/6-311+G(2d,p) (giao, scrf) and Q-Chem. For Gaussian, the solvent setting is scrf=(solvent=chloroform,smd).

For step #2, both agree in the gas phase (to within 3 digits!) but in solvent I get very different answers (Q-Chem is off by ~3ppm in 13C vs Gaussian's 2.1ppm for my dataset.) I think this suggests that I am using the Q-Chem solvent model wrong, which is considerably more complex than what Gaussian seems to expose.

For a start, I have been using:

BASIS  6-311G**

solvent  trichloromethane

in my solvent settings for Q-chem, with the above results. Can anyone recommend a set of Q-chem solvent settings that might yield more accurate chemical shift values and approximate whatever Gaussian's solvent model is doing? I'm also totally open to a different set of best-practices for Q-chem.

  • $\begingroup$ There's a Q-Chem specific forum at iOpenShell - either they, or Q-Chem technical support may have better advice. If so, please post an answer so others can find it in the future! $\endgroup$ – Geoff Hutchison Nov 28 '18 at 16:29

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