MP2.Īfter you have done all the above you can go ahead and calculate NMR tensors. Most appropriate would be perturbation theory, e.g. If you have fully converged geometries at DFT level, you might want to check basic properties with ab initio techniques.
Make sure to run single point calculations first and try optimisations later. BP86 EmpiricalDispersion=GD3BJ, PBE1PBE EmpiricalDispersion=GD3BJ, B2PLYPD3, etc.Īpply solvent corrections for your system, e.g. Validate your geometries with the dispersion correction of Grimme, e.g. #P BP86/Def2TZVPP/W06 DenFit and different functionals, e.g. Validate your geometries with a higher basis set, e.g. You can use density fitting here to speed up your calculation: e.g. Increase the level of theory to pure DFT. For reasonably large molecules a semi-empirical model like pm6 can be used to scan for configurational space. The best way is to start a geometry optimisation on quite a low level. And it might be complicated to find the best (in the sense of most efficient) setup. A few basis set families come to mind: Dunning's augmented family, aug-cc-pVDZ and larger, Ahlrichs and Weigend's def2 family, def2-SVP and larger, and many others, see EMSL.Ĭalculating NMR tensors is always an accuracy versus time tradeoff.
Maybe move away from Pople basis all together. The first thing to do would be to increase the basis set. Using this setup for NMR calculations is worse than guessing. It is called the minimal basis for a reason. (It only describes Pauli correlation, but this is exact.) The next problem is the basis set: It substantially lacks basis functions. The problems start with HF, since it does not describe correlation sufficiently. In short, it fails more often than it is correct.
However, even geometries of well known and/or calculated molecules may be wrong. It is unreliable in many ways, but it is fast. The HF/STO-3G level of theory is what you call the minimised ab initio approach. (This should have been a comment, but it was too long.) While Greg's answer describes sufficiently what the program does, I would like to extend a little on his remark.
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