Asked 42 seconds ago
Hi,
I am facing some problems about the CO polarizabilities calculation with gaussian16 chemistry software:
if I launch a polar CPHF calculation after a opt freq calculation I can obtain alpha and beta,also in the dynamic case, but if I slightly distort the molecule along the normal mode now Gaussian will calculate only the static alpha.
Why is not calculating the dynamic alpha and the static and dynamic beta? I think there may be a problem with the convergence of the polar calculation. How can I fix it ? Maybe I should change the setting for the polarizability calculation?!?
Thank you very much
Francesco
Hello Francesco Ciccarello
In Gaussian calculations for dynamic polarizabilities and hyperpolarizabilities (including β), the software employs different methods, and convergence issues might indeed affect what results you obtain.
Here are a series of steps and settings to check and potentially modify if you are not getting the expected dynamic alpha or beta values after distorting the molecule:
Confirm Method and Basis Set: Ensure that the chosen computational method and basis set are appropriate for polarizability calculations. Some functionals and basis sets may yield better convergence for these properties.
Check Convergence Criteria: Tightening the convergence criteria for the SCF (Self-Consistent Field) could help. Use the SCF=(Conver=6) for tighter convergence criteria or alternatively use SCF=XQC which uses Quadratically convergent SCF.
Frequency Calculation Check: Check the frequency calculation output of the undistorted molecule. If there are any imaginary frequencies, the molecule might be at a transition state or a saddle point rather than a minimum. In that case, polarizabilities may not be meaningful.
Finite Field Approach: If the Coupled-Perturbed Hartree-Fock (CPHF) method does not converge well, you could switch to a finite field approach where you explicitly apply an electric field and calculate the difference in dipole moments to estimate polarizabilities.
Electronic State: Ensure that the electronic state of the molecule is correctly described. If the ground state is open-shell or if there are low-lying excited states, you might need to use methods tailored for such situations.
Input File Check: Re-examine your input file; ensure that the distortion does not inadvertently change symmetry or other parameters that might influence the calculation of dynamic properties.
Polar Keyword: Include the Polar keyword in your Gaussian input to explicitly request calculation of polarizability.
Use of Dynamic Keyword: To calculate dynamic polarizabilities, you must specify the desired frequencies using the Freq keyword in the route section. For example, for dynamic alpha at a frequency of (\omega), use Freq=Wavelength.
Memory and Processors: Insufficient memory or too few processors can sometimes lead to convergence problems. Increase the computational resources if possible.
Check Output for Errors: Sometimes the output file contains warnings or errors related to the calculation that can give you clues about what might be wrong.
Note that these calculations require high RAM powerful systems. In this case you can get help from Molqube.com. Their email address: [email protected]
With luck
Thank you so much for your answer Maryam,
I succeed to solve the problem: I needed to specify only the keyword polar and not also freq,otherwise if the geometry is not a minimum it won't calculate the dynamic polarizabilities.
Still I have another open question,since I am trying to calculate the vibrational dependent first hyperpolarizability.
I found in literature a common approach,but formulas are slightly different one from the other(I report two of theme in attachment), moreover all of them give a results for the first hyperpolarizability components which is different from the gaussian output in the units of measure.
The gaussian output is in C3m3J-2 instead but for example the units of measure of the first formula,even if is graphically reported in the correct way, is in reality C3mJ-1s2.
I report below the papers of the formula.
Article Analyzing the Vibrational Signatures of Thiophenol Adsorbed ...
https://pure.unamur.be/ws/portalfiles/portal/38254125/2018_TetsassiC_these.pdf
Thank you in advance to anyone will answer.
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