On Thu, Apr 20, 2017, Guqin Shi wrote:
>
> As I am reviewing NMA basics, I noticed that in theory, low frequency
> vibration (lower than 500 cm-1) could have non-negligible contribution to
> enthalpy. Cases include torsional vibration, ring bending vibration, etc.
I'm lost here: in the rigid-rotor harmonic oscillator approximation (which
is what makes sense for normal mode thermochemistry) Evib = Hvib, as
vibrations don't contribute to pV (we are also in the ideal gas limit.)
Evib is printed for each frequency in the nmode() output.
If I am missing something here, you would need to provide a precise definition
of what you mean by "Hvib".
Basically, in the MM/PBSA model, the internal energies are made via averaging
potential energies over snapshots; pV terms in the gas phase assume ideal gas
behavior; pV terms in solution are ignored; configurational entropy terms
are estimated via the RRHO approximation. You don't want to try to make
use of Evib since that would amount to double-counting the internal energy
part.
...hope this helps...dac
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Received on Thu Apr 20 2017 - 19:00:02 PDT
