-- ═════════════════════════════════════ Sebastian Kozuch ═════════════════════════════════════ Department of Chemistry Ben-Gurion University of the Negev Beer Sheva, Israel kozuch---bgu.ac.il http://www.bgu.ac.il/~kozuch ═════════════════════════════════════

On Tue, Jun 12, 2018 = at 7:24 AM, Susi Lehtola susi.lehtola[]alumni.helsinki.fi <owner-chemistry]_[ccl.net> wro= te:

Sent to CCL by: Susi Lehtola [susi.lehtola-$-alumni.helsinki.fi] On 06/12/2018 05:27 AM, Guzman, Francisco fg58()njit.edu wrote:

Hello CCL'ers,

Using electron correlation methods, such as CCSD(T), it is a very common ap= proach to extrapolate single point energies to the complete basis set limit= (CBS).

However, is it also common place to apply the the CBS procedure, but to ins= tead use a DFT method such as wb97x, PBE0, or M06-2X?

It's been done in several works, and e.g. Frank Jensen's polarizati= on consistent sets have been constructed so that extrapolation to the compl= ete basis set limit is possible. However, as has already been pointed out b= y other people in the discussion, standard density functionals converge ver= y quickly to the basis set limit - the convergence is exponential as in Har= tree-Fock - that you usually don't need to do any extrapolations to get= a sufficiently accurate number; triple or quadruple zeta is probably alrea= dy converged well enough for any practical purposes.

Extrapolation is typically only necessary if you're trying to nail the = results down to micro- to nanohartree accuracy, which usually isn't ver= y interesting, but may be necessary if one wants to compare results to very= precise real-space calculations. One example is Jensen's recent work i= n J. Phys. Chem. A 121, 6104 (2017), DOI: 10.1021/acs.jpca.7b04760, where h= e shows that using extrapolation one can achieve an accuracy similar to mul= tiwavelet approaches that have recently become tractable for small to mediu= m size molecules.

Note that the Boys-Bernardi counterpoise correction could also be called an= extrapolation method, where the aim is to eliminate basis set superpositio= n error in calculations with small basis sets. This method is extremely com= monly used in applications, as e.g. accurate geometries can be achieved wit= h far fewer basis functions.
--
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Mr. Susi Lehtola, PhD=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0Junior= Fellow, Adjunct Professor
susi.lehtola/./alumni.helsinki.fi=C2=A0 =C2=A0University of Helsin= ki
http://www.helsinki.fi/~jzlehtol=C2=A0 Finland
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Susi Lehtola, dosentti, FT=C2=A0 =C2=A0 =C2=A0 =C2=A0 tutkijatohtori
susi.lehtola/./alumni.helsinki.fi=C2=A0 =C2=A0Helsingin yliopisto<= br> http://www.helsinki.fi/~jzlehtol
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