[gmx-users] PMF in vacuum and pull direction (chris.neale at utoronto.ca)

Eudes Fileti fileti at ufabc.edu.br
Mon Jun 7 14:15:14 CEST 2010


Hello Chris, thanks for the tips, they were very helpful.
Now a new problem appeared. I'm trying to separate the
two benzene molecules from one another while maintaining
the displacement vector aligned with the z axis. For that I
have used "pull_geometry = direction". However, at the end of the
simulation, I notice that the desired alignment (with z) was
not maintained. The molecule is pulled away from z and eventually
crosses the PBC. I took a good look at gmx-list and noticed
that you rode conducting tests with this option. Could you give
me a light on this problem too? Grateful
eef
_______________________________________
Eudes Eterno Fileti
Centro de Ciências Naturais e Humanas
Universidade Federal do ABC — CCNH
Av. dos Estados, 5001
Santo André - SP - Brasil
CEP 09210-971
+55.11.4996-0196
http://fileti.ufabc.edu.br


> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of gmx-users digest..."
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>
> Today's Topics:
>
>   1. Constraint causing system to explode (Warren Gallin)
>   2. PMF in vacuum (chris.neale at utoronto.ca)
>   3. Re: Compile gromacs 4.0.7 with mopac (Stefan Hoorman)
>   4. vibrational spectra of glucose (Nilesh Dhumal)
>
>
> ------------------------------
>
> Message: 2
> Date: Thu, 27 May 2010 12:12:03 -0400
> From: chris.neale at utoronto.ca
> Subject: [gmx-users] PMF in vacuum
> To: gmx-users at gromacs.org
> Message-ID: <20100527121203.orb2sq228s4ow0wg at webmail.utoronto.ca>
> Content-Type: text/plain;       charset=ISO-8859-1;     DelSp="Yes";
>        format="flowed"
>
> Dear Eudes:
> To answer your pbc vs no-pbc question, I suggest that you use pbc=no
> and set nstlist=0 rlist=0 rvdw=0 rcoulomb=0 so that you calculate all
> interactions in direct space with no cutoffs.
>
> ## Major comments that you should still investigate
>
> 1. There is no need to use a virtual atom, the pull code will use the
> center of mass. I strongly suggest that you stop using a virtual atom
> and just use the entire benzene as an argument to the pull code group.
> I have had difficulties with slightly more complicated setups of this
> type.
>
> 2. In test2.jpg, the system without pbc shows a flat PMF after the
> cut-off -- exactly what one would expect. The pbc system shows
> continued interaction -- again what I would expect. So there is
> nothing actually all that strange here. One would not expect to see
> such a drastic difference in a high dielectric such as water, but in
> vacuum I suspect that this is expected.
>
> 3. Please clarify what your cutoff was. I don't see a cut-off listed
> in your .mdp options and leaving this to the default of 1.0 nm is a
> bad idea because it can lead to confusion a times like this. I might
> assume that it was 0.7 nm based on test2.jpg, but then see the point
> #3 below.
>
> 4a. I have no idea what -DPOSRES is actually doing for you since I
> can't see your topology.
>
> 4b. Are you sure that "pull_dim = N N Y" is really what you want?
> Sometimes one wants to average over X and Y, but I am not sure that
> you do in this case.
>
> 4c. What exactly do you believe pull_r0 and pull_r1 are doing for you?
>
> ### More minor notes:
>
> 5. regarding test1.jpg: a PMF is correct to an additive constant,
> meaning that you can shift two PMFs relative to one another. These 2
> PMFs are therefore less different than they appear in your compaison
> plot, but they do differ in the slope between 1.0 - 2.0 nm. This is
> probably just a convergence issue and you will always need to do tests
> like this.
>
> 6. regarding histo.png: can you confirm that the few very short
> gaussians are due to less sampling in a few windows? In any event, the
> overlap looks good.
>
> Chris
>
> -- original message --
>
> Hello Chris, thanks for your attention.
> I'm sending you some links to some tests
> I performed. As I said you will notice that
> depending on the parameter used my simulation
> shows PMF profiles quite different. Especially what
> concerns to the difference between the use or not of the PBC.
>
> https://sites.google.com/site/fileti/files/test1.jpg
> https://sites.google.com/site/fileti/files/test2.jpg
> https://sites.google.com/site/fileti/files/histo.png
>
> I have constructed two very similar topologies (ben-a.itp and ben-b.itp)
> where I put a virtual site in the center of benzene.
> This sites were restrained to keep my molecules
> fixed distance desired.
>
> The basic details of the simulations are given below:1000
>
> define                   = -DPOSRES
> integrator               = sd
> tinit                    = 0
> dt                       = 0.002
> nsteps                   = 5000000 or 500000
> comm-mode                = angular
> nstcomm                  = 1
> comm-grps                = System
> bd-fric                  = 1
> ld-seed                  = 1993
> nstlist                  = 5
> ns_type                  = simple
> pbc                      = no or xyz
> periodic_molecules       = no
> rlist                    = 0
> coulombtype              = Cut-off
> vdw-type                 = Cut-off
> rvdw                     = 0
> DispCorr                 = no
> Tcoupl                   = Nose-Hoover
> tc_grps                  = system
> tau_t                    = 0.1
> ref_t                    = 300
> Pcoupl                   = no
> pcoupltype               = isotropic
> tau_p                    = 1.0
> compressibility          = 4.5e-5
> ref_p                    = 1.0
> constraints              = all-bonds
> constraint_algorithm     = lincs
>
> ; COM PULLING
> pull                     = umbrella
> pull_geometry            = distance
> pull_dim                 = N N Y
> pull_r1                  = 1
> pull_r0                  = 1.5
> pull_constr_tol          = 1e-06
> pull_start               = yes
> pull_nstxout             = 10
> pull_nstfout             = 10
> pull_ngroups             = 1
> pull_group0              = BENa
> pull_weights0            =
> pull_pbcatom0            = 0
> pull_group1              = BENb
> pull_weights1            =
> pull_pbcatom1            = 0
> pull_vec1                = 0 0 1
> pull_init1               = 0
> pull_rate1               = 0.0
> pull_k1                  = 1700 or
>
>
>
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