[gmx-developers] econq settings in constrain()

[Berk Hess]

Hi,

During normal MD the constraint matrix is mass weighted, since the forces
worked on the constraints need be removed, but you are actually constraining
the coordinates or velocities. If you are, in fact, constraining the 
force themselves
the constraint matrices should not be mass weighted.
When doing em the masses in t_mdatoms are all set to 1.
So I guess for your case you can just choose an energy minimizer 
"integrator"
and then things will work correctly.

But actually you can probably directly use the evaluate_energy call in 
minimize.c.
We should really make a Gromacs API that does such things for you,
but we currently don't have the manpower for such projects.

Berk

Peter Matthews Kasson wrote:
> I'm in the midst of an ugly hack of the Gromacs code to calculate forces for an arbitrary set of coordinates.  I've been borrowing from the energy minimization code, but I'm running into a message in constr.c:
>
>  if (econq == econqForce && !EI_ENERGY_MINIMIZATION(ir->eI))
>    gmx_incons("constrain called for forces while not doing energy minimization, can not do this while the LINCS and SETTLE constraint connection matrices are mass weighted");
>
> I'm assuming that's there for a good reason.  Does anyone have a sense for what the econq setting does and what I should set it to in this case?  The constraint code is just a touch hard to follow.  (I could  obviously kudge things so the inputrec looks like an energy minimization run, but I'd like to know what I'm doing first...)
>
> Thanks!
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