# Using double-hybrid functional instead of MP2 to avoid basis set dependence

I'm investigating reaction mechanism and I want to calculate energies of my reagents, products and TS on a higher level of theory than I use for the geometries and frequencies.

I'm thinking about MP2, but I've read that is has significant basis set dependence and it's advised to investigate convergence to CBS limit. Can I use double-hybrid DFT as a substituent, as far as I understand they aren't so basis set dependent?

Is basis set dependence a thing to consider for regular mechanistic calculations that aren't pretending to be quantitative?

• I would still check basis set dependence by at least comparing DZ and TZ. Why do you not want to do it? – Nike Dattani Jan 19 at 15:21
• @NikeDattani is there a formula to estimate MP2 CBS limit, or it's to just see if the energy changes significantly? I have a pretty large system, and such check is twice as much calculations. Also I'm just curious about proper methodology. – roma ichenko Jan 19 at 15:51
• Yes if you have an energy at 2 different basis set sizes, you can extrapolate to CBS using a few different formulas. I understand that doing DZ and TZ is double the number of calculations but DZ is going to cost maybe 10% of what TZ costs, so really it's not much more. I think you have to do TZ anyway because DZ is usually not enough for anything, even with a double hybrid. – Nike Dattani Jan 19 at 15:55
• For converged results, you would need a QZ basis set even for the double-hybrid. As for extrapolating CBS from (DZ, TZ), at least for non-covalent interactions, some authors warn against it, claiming that TZ by itself is actually more accurate. Overall, using a hybrid functional with a significant amount of Fock exchange (because of the TS calculation) may be the more balanced option. Check e.g. the GMTKN benchmarks for ideas which one to choose. – TAR86 Jan 19 at 18:48
• @NikeDattani I've tried DZ and TZ and figured out that MP2/cc-pVTZ calculation doesn't fit my disk quota, so I'll stick to double-hybrid. – roma ichenko Jan 20 at 2:22

The reason to use double hybrids is just to get results of better than MP2 quality at a cost similar to MP2. But, in many cases modern range-separated functionals like $$\omega$$B97M-V out-perform double hybrids at a fraction of their cost...