I have a series of molecules calculated (DFT PBE0-D3/def2-TZVP performed in Q-Chem) and I'm ready to plot PES (Potential Energy Surface). I had to increase the grid to get rid of small imaginary frequencies in frequency calculations.
All calculations were performed with SMD(water) (Geometry Optimization and Frequency calculation), thus, numerical hessian calculation was performed. There are a lot of SCF energies from Frequency jobs that are lower in energy from the final value at "Optimizations converged" step. Does it mean that SCF energy for the "optimized geometry" should be adjusted?
My question is which SCF energy is more correct to use to plot PES and further calculate enthalpy and Gibbs energy of the reactions?

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    $\begingroup$ +1. I am having a hard time though, figuring out what the different SCF energies you're talking about are. You mention SCF energies from frequency jobs being lower in energy than the converged/optimized energies? Then you say there's some intermediate energies that are even lower than the ones from the frequency jobs? What type of SCF is it? HF-SCF or DFT-SCF or CASSCF or something else? Maybe you're saying that the optimized structures have higher SCF energies than non-optimized structures (which could make sense if the post-SCF correlation calculation was lower for the former). $\endgroup$ Commented Jun 14, 2020 at 21:12

2 Answers 2


Generally you want to use the same settings when trying to combine results from different jobs. However, below is the general procedure that I have seen performed for a basic, publishable PES.

1) Perform geometry optimization

2) Perform solvent/frequency calculations

3) Combine solvent energy and enthalpy (from frequency calculation "Total Enthalp...")

4) Use: ΔG = ΔH + TΔS

With that in mind, use the optimized geometry for both the solvent and frequency calculations with the same settings (level of theory, basis set, grid, etc).


I really don't know the output structure of Q-Chem, but in SIESTA, we have the following information:

siesta: Final energy (eV):
siesta:  Band Struct. =   -6333.862502
siesta:       Kinetic =   15927.632813
siesta:       Hartree =   72644.682988
siesta:       Eldau   =       0.000000
siesta:       Eso     =       0.000000
siesta:    Ext. field =       0.000000
siesta:       Enegf   =       0.000000
siesta:   Exch.-corr. =   -8639.068902
siesta:  Ion-electron = -159290.772952
siesta:       Ion-ion =   54887.144618
siesta:       Ekinion =       0.000000
siesta:         Total =  -24470.381434
siesta:         Fermi =      -3.153387

As you can see, there is information about different types of energy, so, the energy to be used in the PES calculation should be the Total energy, as it is the sum of all contributions.


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