[AMBER] QM method used for force constants derivation in GAFF2 ?

From: Marek Maly <marek.maly.ujep.cz>
Date: Tue, 24 Jan 2017 18:47:19 +0100


I have technical question regarding QM methods used in GAFF2 development.

In Amber16 manual, page 286, there is written:

  "We have performed B3LYP/6-31G* optimization for 15 thousands marketed or
experimental ..."

So B3LYP/6-31G* is here QM method used to obtain optimized molecular
structures (low energy
conformations) from which equilibrium geometrical terms (eq. bond lengths,
eq. angles) were obtained
and so in some cases old GAFF values were updated.

But in GAFF2, comparing to GAFF, changed clearly also some force constants.
As an example I can mention c3-c3 bond:

c3-c3 300.9 1.5375 SOURCE1_SOURCE5 88072 0.0058

c3-c3 232.52 1.538 SOURCE1_SOURCE5 88072 0.0058

My question is, which QM methodology was used to derive (perhaps using
paramfit ?) new GAFF2 bond, angle or dihedral force constants ?

I assume, that since here we need just to calculate for each conformation
only it's
  energy (no optimization) and since we also need to obtain reliable force
constants (hence we need accurate QM energies) some more accurate method
was probably used like MP2/6-31G** , MP3/6-31G** etc. to derive QM
energies. Am I right ?

So thank you in advance for QM methodology specification used in GAFF2 for
force constants

    Best wishes,


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