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Hi Sven,


To develop torsion terms for missing terms in GLYCAM06, you will need to generate QM energy profile for that specific torsion.  To do that, optimize the molecule with that torsion restrained in QM (HF/6-31G*), then single point energy calculation with the optimized structure (B3LYP/6-31G*).  We usually vary the torsion angle every 30 degrees.


Once we have QM optimized structures (targeted torsion restrained), perform MM energy minimization with targeted torsion restrained as well.  For parameters you use in MM energy minimization, set V terms for that specific torsion to 0.  You will have the MM energies without rotational energy corrections ("torsion energies").  The differences between QM and MM (without rotational energy corrections) energies are what "torsion energies" are supposed to be.  Then, develop V terms to make sure rotational energies (sum from cosine functions) match the previous energy differences.


It is not complicated, but it will take some time.  The logic behind torsion terms development is that torsion terms (rotational energies) are corrections in MM energy calculations.  Therefore, if the similar terms are fairly small, it means you could either copy from similar terms or set the V terms to 0.  Besides finding similar terms in GLYCAM06, you can also search similar ones in parm and gaff, which are available in AMBER.


Best,


Xiaocong Wang
Complex Carbohydrate Research Center
The University of Georgia
315 Riverbend Road,
Athens, GA, 30602
Tel: (706) 254-7958
E-mail: [log in to unmask]

From: Users of GLYCAM & GLYCAM-Web <[log in to unmask]> on behalf of Sven Hackbusch <[log in to unmask]>
Sent: Friday, September 16, 2016 3:09:18 PM
To: [log in to unmask]
Subject: Re: GLYCAM parameters for 1-Acetylated sugars
 
Hello Yohanna,

Thank you for your offer to help!

I think I will first try to use the approximation you recommend and compare the results to QM computations to verify it is a good enough approximation.
If I understand the Glycam06 paper correctly, I will need to generate a rotational energy curve for the rotation about the Cg-Os bond for my molecule at the B3LYP/6-31++G(2d,2p)//HF/6-31G* level of theory and then to compare this curve to the GLYCAM06 result.
--Can you explain how I generate the rotational energy curve from the GLYCAM06 parameters?

If I were to have to develop my own parameters, would it be correct to use two simple compounds (methyl methoxyacetate and tetrahydro-2H-pyran-2-yl acetate) as a training set to generate the parameters?
Here, I am not completely clear on how the fitting of the parameter terms was performed (in terms of the math involved) and how the rotational energy curve was divided into the respective atomic sequences (as opposed to using just a single atomic sequence in GLYCAM93).

Thank you again!
  -Sven

On Fri, Sep 16, 2016 at 7:17 AM, Yohanna White <[log in to unmask]> wrote:
Hello Sven, 

You have several options for this case. You can either make approximations and make a guess on the torsion terms, develop your own parameters, or we can develop the parameters for you and guide you through it (but this will take us about 6 weeks to get to). 

So if you'd like to make some approximations, you could set  the Os-Cg-Os-C to the same term as a Cg-Cg-Os-C, assuming that if this were a hexane, its torsion term would be similar. And you can set the H2-Cg-Os-C to 0 so it won't have a torsion term. 

Let us know if which plan works best for you. 

Thanks,
Yohanna 

On Tue, Sep 13, 2016 at 9:30 PM, Sven Hackbusch <[log in to unmask]> wrote:
Dear GLYCAM developers and users,

I am interested in simulating a sugar that is acetylated at the 1 position, specifically 1,6-Ac2-alpha-D-Glc, with the GLYCAM forcefield in Amber.
It would appear that there exist no partial charges for this compound (the online builder did not let me place an acetyl derivative at position 1), so I am using the general protocol for deriving the partial charges.
However, in generating the prmtop file using xleap I am encountering the following errors:

 ** No torsion terms for Os-Cg-Os-C
 ** No torsion terms for H2-Cg-Os-C

Presumably, GLYCAM is missing the dihedral angle parameters for the O5-C1-O1-C(acetyl) and H1-C1-O1-C(acetyl) dihedral angles.

Can anyone advise a way to overcome this roadblock?
Thank you very much for your help!

  -Sven


--
Sven Hackbusch
PhD Candidate
Department of Chemistry
University of the Pacific




--
Sven Hackbusch
PhD Candidate
Department of Chemistry
University of the Pacific