3 maj 2007 kl. 15.42 skrev Mark Abraham:
Jiang ran wrote:
Hello everybody,
Thank you all for your suggestions ,
I red the Gromacs software package manual but found that PVT was
not
implemented yet in the Gromacs software package. I want to conserve
both volume and pressure because my professor says that they are both
important for the dynamics of the protein.
I don't think you can set up a non-trivial ensemble where both
pressure
and volume are constant, and even if you can, no force fields have
been
parameterized for it because there's no physical system in which it is
true. Consider an ideal gas in such an ensemble. PV = nRT, so with
P, V
and n constant, then T is also constant, which means your gas
molecules
can only interact in a way such that kinetic energy is constant.
Now for constant n and V you could define negative feedback algorithm
that returned P to a reference pressure, which will force T to vary
inversely. What's the point in doing that?
Actually, Jiang never mentioned constant n. Particle insertion of
some sort could enable the PVT ensemble. Such operations aren't
exactly trivial, however, and could easily mess up the dynamics of
the protein much more than non-constant P or V.
/Erik
So I would like to start
implementing it for the weekend.
I head the idea to introduce langevin dynamics on the atoms close to
the boundary so the magnitude of the random force satisfy the
fluctuation dissipation theorem, however the net inwarded
direction of
the total force on each plane will be solved with a set of langrance
multipliers such that the net inwarded force on the plane gives the
correct reference pressure such that P_ref=-sum(F_plane)/A . In
this
way both volume as pressure are conserved. In which file of the
gromacs
software package I can best start implementing it ?
I started looking in mdrun.c however this didn't make much sence
to me.
There's no shortcut possible here, as I said some other time in the
last
week. Start with the main function and follow through a typical
calculation - even use a debugger to make life easy on yourself.
Learn a
bit about how gromacs works by running a simulation with it first.
Mark
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_______________________________________________
Erik Marklund, PhD student
Laboratory of Molecular Biophysics,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596, 75124 Uppsala, Sweden
phone: +46 18 471 4537 fax: +46 18 511 755
[EMAIL PROTECTED] http://xray.bmc.uu.se/molbiophys
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