[gmx-users] is lincs used with virtual hydrogens?

[chris.neale at utoronto.ca]

Thank you Roland.

I did use:

constraints = all-bonds
lincs-iter =  1
lincs-order =  6
constraint_algorithm =  lincs

 From looking at the manual, I figured that angle and bond constraints  
would all be done by LINCS if I had done (A):

pdb2gmx -vsite none
constraints = h-angles
(a combination that I have never tried)

But when I use (B):

pdb2gmx -vsite hydrogen
constraints = all-bonds

It seems possible to me that LINCS is not used but instead the  
position of the atom is simply built from a mathematical function.  
Perhaps this all stems from my lack of thorough understanding of  
LINCS, but it seems to me that there need be no iteration to simply  
place an atoms based on virtual_sites3 (which are constructed by  
pdb2gms -hydrogen)

For now, I'll simply add a line to state that I built virtual sites  
for hydrogen atoms to make it clear, but I'd still like to understand  
the difference between options A and B, above, if you have some time.

Thank you again,
Chris.





On Sat, Jun 18, 2011 at 4:13 PM, chris.neale at utoronto.ca <
chris.neale at utoronto.ca> wrote:

> Dear Users:
>
> If I create the topology of a peptide like this:
>
> pdb2gmx -f protein.gro -vsite hydrogens
>
> And then simulate it in vacuum, is lincs used at all? I believe that
> it is, as if I use a timestep that is too large then I get LINCS
> warnings about angles rotating more than 30 degrees, but that warning
> message could possibly have been written with the assumption that I
> used LINCS and not virtual hydrogens.
>
Probably. To make sure check the constraint-algorithm selected in your mdp.
BTW: If you want to use large timecheck you should normally use
constraints=all-bonds and lincs-order=6.


>
> Finally, is there a method that needs to be named or cited in relation
> to the fact that the angles are now constrained? Is that also done
> with P-LINCS?
>
This is also done with P-LINCS. Not sure whether one should sign something
regarding the construction/usage of v-sites.

Roland


>
> Thank you,
> Chris.