CCL:G: All electron basis set for H2Te



 Sent to CCL by: Susi Lehtola [susi.lehtola%x%alumni.helsinki.fi]
 On 1/21/19 2:27 PM, Mo Fateh mo.fateh+/-yahoo.com wrote:
 
 Sent to CCL by: "Mo  Fateh" [mo.fateh]_[yahoo.com] Dear CCL
 Subscribers,
 I am going to do geometry optimization for H2O, H2S, H2Se, and H2Te
 using Gaussian 09. What is the suitable All electron basis set for
 optimization with B3LYP?
 I have checked the Gaussian website and found that the following bais
 sets are suitable:
 1- DGDZVP basis set 2- QZVPand Def2 3- UGBS basis set
 Which is one you recommended? and Why? Please suggest any other
 suitable basis set
 
 First, do you really need an all-electron treatment? Using an effective
 core potential (ECP) typically gives you good results with a low cost.
 The def2 sets are not all-electron, but use relativistic core potentials
 starting from the 5th period.
 In addition to def2, you can also use the correlation consistent series,
 cc-pVXZ-PP, for the heavy atoms, which employs ECPs. These are probably
 not in the code you are using, but you can download the sets from the
 Basis Set Exchange (BSE) at https://bse.pnl.gov/bse/portal.
 If you want to do all-electron calculations, then you have to include
 relativistic effects to get any kind of reasonable accuracy. It looks
 like Gaussian can do second-order Douglas-Kroll-Hess (DKH2)
 calculations, and that the program uses a finite nuclear model in such a
 case. But, when you do a relativistic calculation, you have to make sure
 that the basis set has actually been designed for the method you are using.
 Unfortunately, there aren't many relativistic basis sets available on
 BSE. IIRC, the ANO-RCC set by Almlöf, Roos, and coworkers available on
 BSE has been parametrized with DKH2, so that would be an okay choice.
 Then, there are the x2c-SV(P)all, x2c-SVPall, x2c-TZVPall and
 x2c-TZVPPall basis sets by Pollak and Weigend, which are all-electron
 versions of the def2 basis sets; however, these basis sets have been
 formed with the eponymous X2C approach which differs from DKH2, and so
 the contractions may not be right for DKH2.
 If you want to play on the safe side, you could decontract these sets;
 like I implied before, the main problem with different relativistic
 models is that the core orbitals may be dissimilar. Using a fully
 uncontracted basis set takes away this problem; of course, then your
 basis set becomes larger.
 Personally, I would recommend either using ECP sets like def2, or if you
 really need all electrons, then I'd go with the x2c series. These are
 both commonly used, so I'd expect fewer problems.
 UGBS is a fine basis set for mean-field calculations on atoms. The
 problem is just that it is pretty huge (71, 84, 163, and 179 for O, S,
 Se, and Te, respectively), while it lacks polarization and correlation
 functions altogether. This means that calculations in molecules and/or
 at a post-HF level of theory will be unbalanced; you could trade off a
 small decrease in accuracy for atoms for a large improvement for
 molecules with the same number of basis functions by using a different
 kind of basis set.
 There are variants of UGBS with polarization functions, but
 1) these are much larger than even the plain UGBS basis set; several
 hundred functions per atom,
 2) I haven't been able to find any description on how the polarized
 basis sets have been formed, and
 3) I am not aware of any accuracy benchmarks with these sets.
 --
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 Mr. Susi Lehtola, PhD             Junior Fellow, Adjunct Professor
 susi.lehtola-*-alumni.helsinki.fi   University of Helsinki
 http://susilehtola.github.io/     Finland
 ------------------------------------------------------------------
 Susi Lehtola, dosentti, FT        tutkijatohtori
 susi.lehtola-*-alumni.helsinki.fi   Helsingin yliopisto
 http://susilehtola.github.io/
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