amber torsion energy

[Letif Mones]

Hi,

I did a molecular mechanical calculation in cp2k using an amber topology file (force field: amber99). When I checked the bonded energy terms I found that the bond and angle terms are correct but the dihedral energy is twice as the energy calculated by sander. I think the reason is that in the amber FF the form of the torsion potential is: K / 2 * [ 1 + cos[ M*PHI - PHI0 ] ] while cp2k uses the form of K * [ 1 + cos[ M*PHI - PHI0 ] ].
So should not the torsion force constant read from an amber topology file be multiplied by 0.5 (and the improper one too as it has the same form)?

Letif

[Teodoro Laino]

Dear Letif,

thanks for reporting this issue. All parameters should be converted at the moment they are read. We also had few examples to check the numerics and I remember they were matching with Sander they were OK.
Do you have an input file (including all relevant files) that shows this problem?

Thanks,
Teo

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[Letif Mones]

Hi Teo,

I am sending the following files: amber and cp2k inputs, amber restart file and amber topology file (dmpoh_vacuum.atop). I also attached another topology file (dmpoh_vacuum_mod.atop) in which I halved the dihedral force constants. With this latter cp2k gives the right answer.

Regards,

Letif

[Teodoro Laino]

Thanks Letif and apologies for the delay.
The issue was indeed a bug and the fix is in the SVN. Should you encounter other discrepancies, please, do not hesitate to report them.
best,
Teo

> <amber.inp><cp2k.inp><dmpoh_vacuum_mod.atop><dmpoh_vacuum.atop><dmpoh.rst>--

[Ajay Muralidharan]

Hello!

Seems like TORSIONS is still broken in svn:18464

I think this might be due to moving from Amber ff99SB to Amber ff14sb.  

Check out:

https://groups.google.com/forum/#!msg/cp2k/_uaFXr1U3J4/428AZa87BQAJ

-Ajay