AMBER Archive (2006)

Subject: Re: AMBER: H-Bond Terms in amber force field

From: Jiri Sponer (
Date: Wed Nov 15 2006 - 17:20:18 CST

Dear Ilyas:

   the problem most likely comes from the implicit model.
   (Stability of your GGCC with implicit model does not mean
   that it is correct in all aspects, it just does not fall
   appart in your particular simulation).
   You need to be exceptionally careful with implicit models
   and do not expect high accuracy, there are numerous fundamental

   Explicit H-bond term to the best of my knowledge is not needed
   and amber, when charges are consistently derived, match
   close to perfectly MP2 complete basis set + CCSD(T) correction
   data for stacking and base pairing unless bases are formally
   There obviously are molecular orbital contributions to H-bonding
   and stacking not included in forcefields but these do not
   cause major troubles in standard simulations and would not be included
   via the old H-bond term.
   (Actually, if this is H-bond error, it would fall appart
   with explicit solvent, too).

   Best wishes Jiri

> Dear All,
> I have a system which is formed by unnatural base pairs; it is a
> (5'iGiGiCiC3')_2 helix. This helix is more stable than (5'GGCC3')_2
> structure. This is experimentally verified.
> When I use implicit solvent model to simulate both these structures, GGCC
> stays helical while iGiGiCiC falls apart.
> Amber force field does not have an explicit term for H-bonds in the
> potential energy function. In the old amber force fields, there is an
> explicit H-bond term present in the function. I wonder why the H-bond term
> is taken away from the force field, and how the 'H-bond' term is
> incorporated into the new amber force field. This might help me to figure
> out why iGiGiCiC is falling apart and how I can solve this problem. Thanks
> in advance for any suggestions.
> Best,
> --
> Ilyas Yildirim
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> - Department of Chemisty - -
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