Re: Question on charges in CVFF on CCL

[Ken Butenhof <kenb-0at0-accelrys.com>]

 Hello CCL'ers,

 In Insight II for all force fields except CFF, assignment of charges (and
 atom types) to each atom is done with the "Potentials"
 parameter block, which is accessed from the "FF" icon. 
 The Insight program assigns atom types and partial charges to each atom
 in the structure based on information in a residue library file (.rlb) if
 the residue is known or (if not found in a residue library file) from the
 bond increments found in the force field file based on the atom
 types.  The charge values present in the residue library file and
 the bond increments in the force field file 
 (.frc) are derived from experimental (usually crystallographic) data and
 quantum mechanical modelling.  The purpose of using a bond increment
 scheme for residues not in the residue library is that the local bonded
 environment (as described by the atom types) determines the atom centered
 charge.  The Gasteiger method first determines the overall charge of
 the system and then distributes that charge throughout the molecule (the
 Gasteiger method is available for use with Discover in Cerius2). 
 Note that CVFF was developed to reproduce peptide and protein
 properties.   The CFF force field has far greater support for
 highly funtionalized small molecules.
 This information is available to all Accelrys users at
 http://www.accelrys.com/doc.

 Ken

> Dear all the CCLers:
>      I am a user of InsightII of Accelrys(former MSI). When
>  I perform
>  comutations, such as molecular minization or dynamics,I often use CVFF
>  force field. I always
>  use FF (ForceField). This command is activated on the left side by
>  clicking on the FF
>  button. Then select Forcefield/Potentials and make sure that all of the
>  settings as
>  follows.
>             
>  Potential Action [Fix]
>
>             
>  Partial Charge Action [Fix]
>
>              Formal
>  Charge Action [Fix or Accept]
>
>      My question is how the partial charges of my molecules
>  are computed?
>  In my opinion,the partial charges are part of the CVFF force field. When
>  CVFF
>  is used, thepartial charges are automatically loaded from the system
>  file. This is
>  quite different from MM2 (Tripos) method which often calculate partial
>  charge through
>  Gasteiger-Huckel method for small molecules.Is this so? I have referred
>  several papers,
>  but find no how the charges are computed.

Kenneth Butenhof, Ph. D.        phone: 800 756-4674
Principle Scientist             email: kenb-0at0-accelrys.com
Life Science Support            web:  www.accelrys.com/support
Accelrys