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Dear Tian,
I would like to report a reproducible problem that we have encountered in ORCA 6.1.1 and would be very grateful for your advice.
Our study concerns a series of Lewis acid–base complexes formed between Group 13 Lewis acids and the nitrogen atom of a six-membered nitrogen-containing ring (CA). The systems under investigation are
• BF₃–CA
• AlF₃–CA
• GaF₃–CA
• InF₃–CA
• TlF₃–CA
where "CA" denotes the nitrogen-containing six-membered ring used throughout our study.
The final goal of this project is to calculate adiabatic electron affinities for these complexes. Therefore, diffuse basis functions are essential for obtaining reliable energies of both the neutral and anionic species.
To construct the augmented basis, we did not use arbitrary diffuse exponents. Instead, we generated the additional diffuse functions using the even-tempered procedure, and only one additional diffuse S shell and one additional diffuse P shell were added to each element. The same augmentation strategy was applied consistently to all atoms.
The computational protocol was as follows:
1. The lowest-energy conformer was first located using the xTB GOAT conformer search.
2. The lowest-energy conformer obtained from GOAT was then fully optimized using ORCA.
3. Vibrational frequency calculations were subsequently performed.
This protocol works perfectly for BF₃–CA, AlF₃–CA, GaF₃–CA, and InF₃–CA.
However, the TlF₃–CA complex behaves differently.
When the additional diffuse functions are included, geometry optimization converges normally, but the subsequent frequency calculation always produces a single low-frequency imaginary mode (approximately −20 to −30 cm⁻¹).
If the additional diffuse functions are completely removed, the imaginary frequency disappears and the optimized structure becomes a true minimum.
Unfortunately, removing the diffuse functions is not an acceptable solution for our project because the final objective is the calculation of adiabatic electron affinities, where diffuse functions are required.
During the past several days we have carefully investigated this problem and tested many different approaches.
The following attempts were performed:
• Performing a full TightOpt + Freq calculation.
• Performing geometry optimization and frequency calculation as two completely separate jobs.
• Starting from the optimized geometry written in the final XYZ file.
• Using analytical Hessians during optimization.
• Recalculating the Hessian repeatedly during optimization.
• Increasing the maximum optimization iterations.
• Using tighter SCF convergence.
• Enforcing SCF convergence (ConvForced true).
• Using DEFGRID3 integration grids.
• Carefully checking SCF convergence and optimization convergence.
• Repeating the calculations from scratch.
• Verifying that the optimized geometry is reproducible.
In addition, we carefully investigated whether the imaginary frequency originated from an unfavorable initial geometry.
We manually displaced the structure several times along the imaginary mode, generated modified geometries, reoptimized these structures, and repeated the frequency calculations. We also introduced several reasonable structural perturbations and repeated the complete optimization procedure.
Nevertheless, after every optimization the same low-frequency imaginary mode reappeared.
Therefore, the problem does not appear to originate from an unfortunate starting geometry or from being trapped in an obvious local minimum.
At present, this behavior appears to be specific to the TlF₃–CA complex, since all other Lewis acid complexes in the series behave normally under exactly the same computational protocol.
Because the issue only appears when diffuse functions are added to the Tl-containing system, we are wondering whether this could be related to one or more of the following:
• the treatment of very diffuse basis functions on heavy elements,
• the X2C relativistic Hamiltonian,
• Hessian construction,
• numerical integration,
• optimization algorithms,
• or another aspect of ORCA's implementation.
For your convenience, we have attached a ZIP archive containing all relevant files, including:
• input files,
• output files,
• optimized geometries,
• Hessian files,
• GBW files,
• and any other files that may help reproduce and investigate the problem.
If this behavior is expected, we would greatly appreciate an explanation.
If it is not expected, we would be grateful if you could indicate whether this may represent a numerical issue or an implementation issue in ORCA.
Finally, if there is any ORCA-specific solution to this problem—such as using additional keywords, SCF options, geometry optimization settings, Hessian-related options, integration grid settings, or any other input blocks recommended by the ORCA developers—we would greatly appreciate it if you could provide a complete working ORCA 6.1.1 input file (not just isolated keyword suggestions).
Since this computational protocol will be applied consistently to the entire series of Lewis acid complexes (BF₃, AlF₃, GaF₃, InF₃, and TlF₃), we would prefer to use an input that follows the officially recommended ORCA methodology for such systems. If any modification of our current input is advisable, please rewrite the entire input file accordingly.
Thank you very much for your time and your assistance.
We look forward to your advice.
Kind regards,
Saeed
Last edited by saeed_E (2026-07-26 09:44:58)
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Dear Tian,
The attached file includes the final and seemingly reasonable and scientific justification for the observed low negative imaginary frequency for TlF3...CA complex. Such a justification is based on the PES scan exactly along the negative normal mode using Gview.
I would be highly grateful if you kindly have a look and let me know your opinion.
Sincerely,
Saeed
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