[gmx-developers] Calling spread_vsite_f twice.

[Lee-Ping]

Dear all,

I wish to call "spread_vsite_f" multiple times in a development version
of GROMACS, in which I have spent the last few years implementing an
automatic force field parameterization code.  

My main question is:  What variables are modified by spread_vsite_f,
other than f itself?  If I call spread_vsite_f several times, do 'bad
things' happen?

My reasoning for wishing to do this is as follows.

My subroutines compute first and second derivatives of both the energy
and the force with respect to the force field parameters.  (This is
computationally costly, but this is for reparameterizing a force field,
not running dynamics.)  The first derivatives are stored in a (P*3N)
array where P is the # of parameters and N is the # of atoms.  I plan to
run serial jobs only, so there is no need to worry about domain
decomposition or anything like that.

I would like to parameterize a four-site water model, and this requires
that I compute derivatives of the forces on the interaction sites which
are "spread" over the atoms.  I intuitively expect that the force
derivatives would be spread over the atoms in the same way as the forces
themselves, and thus I should be able to call the spread_vsite_f
function but substitute the "f" variable with the appropriate derivative
"f^p".  I know that spread_vsite_f modifies fshift as well, so I will
replace it with a dummy.

However, I don't know whether spread_vsite_f changes any of the other
variables that it is called with (i.e. gmx_vsite_t *vsite, t_idef *idef,
etc.)  I am worried that spread_vsite_f may do some one-time work (or
rather, once per force evaluation), and calling it multiple times in a
single force evaluation may lead to unintended consequences.

After I get this to work, eventually I would like to extend the force
matching procedure to the virtual site parameters themselves .. that's
in the future though. :)

Your input is much appreciated.  Thank you!

Lee-Ping Wang
MIT Department of Chemistry