On Feb 1, 2009, at 8:22 PM, Mark Abraham wrote:
Joshua Ballanco wrote:
On Feb 1, 2009, at 6:48 PM, Mark Abraham wrote:
Joshua Ballanco wrote:
Hi,
I'm attempting to model a system involving a small DNA 3-mer.
Without any explicit constraints, the helix begins to come apart
around 0.75 ns to 1 ns into the simulation.
Presumably you have a 3-mer of helices, of which at least one
comes apart. Does a single helix in water survive? (Giving a
better description of your simulation system would be a good idea!)
Apologies for not being more descriptive... It is a single strand
of DNA containing 3 A-T base pairs. The system also contains a
single Arginine residue. Simulating it in water leads to the single
DNA strand gradually coming apart. With the DNA and water alone,
the helix stays together much longer, but still eventually comes
apart.
OK, so your model physics for DNA is intrinsically broken. Where did
you get it?
The coordinates are from 3DNA. I'm using the terms from the G53a6
force field for DADE and DTHY. As for the H-bond physics, I've thus
far been unable to find a good suggestion for how to handle these
explicitly using Gromacs. Do you have a recommendation as to where I
should be looking? (None of the primary literature I've looked through
thus far seems concerned with MD of such short DNA fragments).
So I'm now attempting to add restraints for the base-pair H-
bonds, but I'm having trouble. It seems like no matter what I
try, my system reliably explodes within the first 1 ns. My
constraints look like this:
[ distance_restraints ]
; ai aj type index type’ low up1 up2 fac
18 136 1 0 2 0.0 2.0 2.1 1.0
14 134 1 0 2 0.0 2.0 2.1 1.0
43 114 1 0 2 0.0 2.0 2.1 1.0
39 112 1 0 2 0.0 2.0 2.1 1.0
68 92 1 0 2 0.0 2.0 2.1 1.0
64 90 1 0 2 0.0 2.0 2.1 1.0
I've tried pre-equilibrating for up to 100 ps, but even that
doesn't prevent the system from eventually exploding.
Your .mdp settings for distance restraints may also be relevant
here - not least in setting the existence and magnitude of these
restraints.
As I understand, the only relevant lines are:
constraints = all-bonds
integrator = md
disre = simple
disre-fc and others are also relevant. See manual chapter 7.
Thanks for the pointer. I had overlooked most of the options there,
since I'm not actually doing anything related to NMR. (That'll teach
me to read more carefully!) Unfortunately, playing around with this,
disre-tau, disre-weighting, and the weighting factors for each bond
have not, so far, avoided the explosion.
For PME I was using:
coulombtype = PME
rlist = 0.55
rcoulomb = 0.55
rvdw = 0.55
fourierspacing = 0.1375
I agree with Justin that these are very weird for normal usage.
Thanks for pointing that out. I'm relatively new with Gromacs, and
hastily reduced these values to fix the relatively small box my system
fits in. I doubled the short box dimension (triclinic; was --> 2.0,
2.1, 1.1 now --> 2.0, 2.1, 2.0) and increased the radii to the (as far
as I can tell) more recommended values:
coulombtype = PME
rlist = 0.9
rcoulomb = 0.9
rvdw = 0.9
fourierspacing = 0.12
Unfortunately, even with all of these changes, I'm still getting an
explosion (and my simulation is quite a bit slower).
Thanks again for the pointers. I'm going to try running everything
with ffamber to see if it does a better job with the DNA (without the
added restraints). I presume the port validated with Gromacs 3.3.1 is
still good for 4.0?
Thanks!
- Josh_______________________________________________
gmx-users mailing list [email protected]
http://www.gromacs.org/mailman/listinfo/gmx-users
Please search the archive at http://www.gromacs.org/search before posting!
Please don't post (un)subscribe requests to the list. Use the
www interface or send it to [email protected].
Can't post? Read http://www.gromacs.org/mailing_lists/users.php
