CCL: Correction on single-point energies



 Sent to CCL by: Eric Hermes [erichermes-x-gmail.com]
 Ivan,
 There is an argument to be made whether the gas-phase or solution-phase
 frequencies are more appropriate to use for the calculation of free
 energy of a species in solution, but I will not comment on which
 approach is "better" or "more physical".
 However, frequencies calculated at anything other than the minimum-
 energy geometry are not physically meaningful. While you have minimized
 the geometry on your BP86/6-31G**/LANL2DZ/gas-phase potential energy
 surface, this is not guaranteed to be (and in general is not going to
 be) a minimum-energy geometry on the M06L/6-311G**/SDDALL/toluene
 potential energy surface. So, performing a frequency calculation on the
 latter system without relaxation is not going to result in physically
 meaningful frequencies. If you are concerned that frequencies
 calculated with the lower level of theory are going to be
 insufficiently accurate, you will need to precede any frequency
 calculations at a higher level of theory with a geometry minimization
 at that same level of theory.
 Eric
 On Wed, 2017-06-14 at 11:35 -0400, Ivan Franzoni
 ivan.franzoni[A]utoronto.ca wrote:
 > Sent to CCL by: "Ivan  Franzoni" [ivan.franzoni *
 utoronto.ca]
 > Hello!
 >
 > I am computing the energy of a transition-metal catalyzed organic
 > reaction.
 > I optimized all the intermediates and transition states in gas-phase
 > at the
 > BP86/6-31G**/LANL2DZ level of theory and performed frequency analysis
 > to
 > confirm the nature of all stationary points. I next run single point
 > calculations in toluene at the M06L/6-311G**/SDDALL level of theory
 > and
 > also repeated a new frequency analysis.
 > I saw that someone uses energy and zero-point, enthalpies and free
 > energies
 > corrections from the SP calculation in solvent. In other cases
 > someone uses
 > the total energy obtained in solvent and add the correction
 > previously
 > obtained in gas-phase (so no frequency analysis on the single point
 > calculation).
 > Can anyone comment of these different approaches and maybe suggest
 > which
 > one is the best?
 >
 > Thank you,
 >
 > Ivan Franzoni
 > UofT
 >
 >
 >
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