wjc404 发表于 2023-2-5 21:40 Dear wjc404, Thank you very much for your valuable effort. |
七尺贱 发表于 2023-2-5 20:39 The current topic is quantum chemistry rather than wavefunction analysis or Multiwfn. Discussing in this forum is fully appropriate. |
本帖最后由 wjc404 于 2023-2-16 18:38 编辑 maximos 发表于 2023-2-3 00:33 The static correlation effect of the TS structure seems moderate. (Some groups far from the reaction site are replaced by H atoms to make the calculations affordable. ) (1) The TS structure was optimized at B3LYP-D3/def-TZVP level with IEEPCM solvation model (in dichloromethane). The optimized geometry is given below.
(2) The stable UHF wave function of the above structure was obtained (S2=0.847). It was used as the initial guess for a Brueckner-CCD calculation. The latter yielded a reference wave function with <S**2> lower than 0.001, indicating the spin polarization has been removed. The maximum double excitation amplitude from that reference is 0.079. (3) The natural orbitals at Brueckner-CCD/def-TZVP level were then generated. The result can be downloaded from https://pan.baidu.com/s/1nG6kRcrTkE75dW5Xoqk8Ew with extraction code bkxa. From the occupation numbers (each ≤0.10 or ≥1.90) we can see that this structure is not strongly correlated. Since the multireferential character is not significant, DLPNO-CCSD(T) should be OK here for single-point energy correction of TS. |
LiHuaYu 发表于 2023-2-5 13:18 游客，本帖隐藏的内容需要积分高于 25 才可浏览，您当前积分为 0 |
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What I can see is the TS was located in gas phase. Is it the condition that experiment was carried out? I think you should strongly consider factors that @sobereva mentioned: solvent effects and thermodynamics. Your system is not that big, so I suggest that electronic energies need to be improved based on your final TS geometry. Def2TZVPD or a bigger basis set is highly recommended here. You also might need to pay attention to standard condition (1M) if you would like to identified your TS in a solvent. Here is my favorite solvent model: scrf(smd, solvent = your_solvent). |
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The quality of 6-311G(d,p) is insufficient. I suggest using def2-TZVP for evaluating single point energies based on your optimized geometries. Contradiction with experimental observation is not always caused by the choice of theoretical method, you should also pay attention to other factors, especially thermodynamical correction (Gibbs free energy should be used instead of electronic energy) and solvent effect (solvation model should be employed). |
wzkchem5 发表于 2023-2-2 18:47 I uploaded a transition state of the system as gjf file to see if my chosen level is suitable. 
TS.gjf
(3.49 KB, 下载次数 Times of downloads: 14)
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maximos 发表于 2023-2-2 11:12 You can use ORCA for free as long as you are an academic user, and ORCA is easy to learn, particularly if you only use a few features of ORCA. This is unlike Gaussian, which comes at a price. Within Gaussian you may be able to do a brute-force B2PLYP calculation, for example. But that only works when your system is single-referential. If your system is multireferential, then sooner or later you'll have to switch to ORCA or Molpro etc. |
wzkchem5 发表于 2023-2-2 17:45 Dear wzkchem5, Thank you very much for your reply,but, all the alternatives you mentioned is out of my reach, due to i only can use Gaussian 09 and the modest computational resources. I work with molecules contain at the most 50 atoms, Is there another alternative to overcome this issue ? |
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Please note that wB97XD and M06-2X are not necessarily better than B3LYP. Indeed, the former two have higher amounts of HF exchange, which make them more suitable in cases where self-interaction error is important (e.g. radical ions, charge transfer). However, this also makes them less capable of describing non-dynamic correlation, and fail for systems where static correlation is important (B3LYP is also unreliable under these cases, but usually less so than functionals with higher amounts of HF exchange). I would recommend you use a method that is fundamentally better than the methods that you are using. If your system is indeed manifestly multi-referential, then NEVPT2 may be a good idea. Otherwise, DLPNO-CCSD(T) can generally be recommended. If DLPNO-CCSD(T) is not affordable, then double hybrids like PWPB95-D3 may also be considered. |
sobereva 发表于 2023-2-2 16:36 I'm so sorry for the inconvenience, I modified the post. |
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