Hi, everybody,
I am a freshman for using GROMACS. I used to do simulations using
Materials Studio. Now I notice that versatile analysis tools are
present in GROMACS, so I decide to turn for GROMACS. I have built
some polymeric structures using the Amorphous Cell module in
Materials Studio4.0. Can these structures be directly input into
GROMACS for a further simulations? How can I do that? Can you give
some hints?
If you only want to use gromacs for analysis, then this can be done
relatively easily. If, however, you want to use gromacs to continue
simulations then it will be quite a bit harder.
I have never actually tried the procedure that I outline below, but I
am fairly sure that it will work. At least I am convinced that, if it
works, the analysis will be correct with the exception of analysis
programs that do mass weighting, electron density calculation, or
other charge-based calculations. If you want those things to work
properly too then you will need to be more careful about the atom type
and/or charge that you apply in the .rtp file (see below).
For the case of analysis first:
Output your MS4 trajectory as a single pdb file with all of the frames
in one file (you'll probably need to use MS4 to do that (call this
file traj.pdb). This can then be input to gromacs analysis tools. Also
create a single frame pdb from the first frame (call this file
first.pdb).
You will need a topology file in order to carry out analysis. However,
for this purpose, the parameters don't need to be correct or even
exist. All you need is the bonding profile and atom names. For the
purpose of an example I have created an entry for molecular oxygen to
a .rtp file:
[ oxygen ]
[ atoms ]
;name type charge charge_group ;;this is a comment line
O1 opls_154 0.0 1
O2 opls_154 0.0 1
[ bonds ]
O1 O2
Take a look at some of the amino acid examples to see how residues are
bonded to each other so that you can define monomeric building blocks.
Don't use this for simulations, but it should allow you to carry out
analysis on trajectories that you already have. I chose opls_154 as an
existing oxygen atom type, but as long as the type exists I don't
imagine that it even needs top match the type of atom (i.e. you could
use opls_154 as the atom type for every atom in your entire molecule).
Assign all charges as zero, and put them all in a single charge group.
For a more complicated / realistic example take a look at the ethanol
entry in the gromacs/top/ffoplsaa.rtp file, or the entries for amino
acids in that same file.
Once you have correctly created your .rtp entry and placed it at the
bottom of gromacs/top/ffoplsaa.rtp, then run these commands:
pdb2gmx -f first.pdb -ff oplsaa -p topol.top
touch empty.mdp
grompp -f empty.mdp -c first.pdb -p topol.top -o analysis.tpr
You will need to use the -maxwarn input with a large number probably
because a lot of interactions will not end up being specified.
You now have a tpr file that can be usef for analyzing your traj.pdb file.
##########
If you want to actually continue your run then you need to do the same
thing, but the charges and atom types need to be correct. Also the
ffoplsaanb.itp and ffoplsaabon.itp files need to contain parameters
for all types of interactions for your specified atom types. Further,
you will need to specify your improper dihedrals explicitly in the
.rtp file.
Note that for actual run continuation this is a huge amount of work. I
don't reccommend it for a first time gromacs user.
##########
The actual parameters are not going to matter for analysis of
trajectories that you already have. However, for continuation of
simulations, please note that it is possible to make up a topology
that runs without error and does not crash and yet gives you totally
bogus results.
I realize that this post is a little scattered. If you do decide to go
this route, please use the search feature of the mailing list (don't
forget to check the mailing list box below the search button) and the
manual to help you figure out what .rtp files are etc.
Chris.
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