Dear Amanda,
Can you clarify the shape of the bilayer you are using as a starting
structure? As far as I know, Amber only supports square or rectangular
bilayers. A system snapshot, before and after running in Amber, would be
helpful. You might send it to me individually, if not to the list.
I do not anticipate a problem with using ntp=3 (semiisotropic scaling)
with barostat=2 (MC barostat), but this combination has not - to my
knowledge - been rigorously tested. My group has tried it, with csurften=3
(constant surface tension in xy plane) and gamma_ten=0.0 dyne/cm. This
effectively couples the x and y dimensions, preventing box shape change in
long simulations that can be problematic, especially using the MC barostat.
We seem to get more or less the same physical behavior using this approach,
compared with ntp=2 (anisotropic scaling), which has also not been tested
rigorously with bilayers, to my knowledge. However, applying a surface
tension of zero may not be entirely rigorous scientifically. Though the
surface tension of perfectly symmetric bilayers "should" be zero, there are
likely fluctuations and transient asymmetries that may be important for
some research problems.
Note that Lipid14 - and possibly also later Amber lipid force field
versions - were validated and tested with the Berendsen barostat
(barostat=1). I do not know what barostat was used for Charmm36m. The MC
barostat does, in our experience, change the bilayer properties of Lipid14
POPC somewhat, with a tendency to reduce the area per lipid and increase
the thickness (unpublished results). We find these variations to be within
experimental error for area per lipid but possibly a bit high (by ~1
angstrom) for bilayer thickness. My group does routinely use the MC
barostat with anisotropic scaling for bilayer simulations, but there are
definitely accuracy considerations that you would want to monitor. I
believe Rich Pastor at NHLBI has done some recent testing of a Charmm lipid
force field with the MC barostat, but I do not think the work has been
published yet.
Best,
Sally
Sally Pias
New Mexico Tech
sallypias.gmail.com
On Wed, Jun 3, 2020 at 6:31 PM Amanda Buyan <Amanda.Buyan.anu.edu.au> wrote:
> Dear Matias,
>
> Not a silly question at all! I have just checked it, and there doesn't
> appear to be any gaps or spaces between the lipids in the periodic images,
> so I don't know if "excessive" space is an issue, but it is helpful
> eliminating one possibility!
>
> Cheers,
> Amanda
>
> ________________________________
> From: Matias Machado <mmachado.pasteur.edu.uy>
> Sent: 04 June 2020 00:51
> To: AMBER Mailing List <amber.ambermd.org>
> Subject: Re: [AMBER] CHARMM Forcefield in Amber - which barostat and ntp
> to use?
>
> Dear Amanda,
>
> A very silly question, have you checked the PBC condition before running
> the simulation? I mean, have you visualized the periodic images of the
> membrane before any simulation?
>
> I don't know how CHARMM-GUI prepares the system for running in AMBER code,
> but there are some issues when setting the periodic box in LEAP for
> membranes which leads to "excessive" space between lipids of neighboring
> images breaking the continuity of the membrane. That kind of issue may
> create odd behaviors at edges...
>
> Hope it helps...
>
> Matias Machado
>
> ------------------------------------
> PhD.
> Researcher at Biomolecular Simulations Lab.
> Institut Pasteur de Montevideo | Uruguay
> [http://pasteur.uy/en/labs/biomolecular-simulations-laboratory]
> [http://www.sirahff.com]
>
> ----- Mensaje original -----
> De: "Batuhan Kav" <bkavlist.gmail.com>
> Para: "AMBER Mailing List" <amber.ambermd.org>
> Enviados: Miércoles, 3 de Junio 2020 10:44:06
> Asunto: Re: [AMBER] CHARMM Forcefield in Amber - which barostat and ntp to
> use?
>
> Hello,
>
> I don’t have access to the data anymore but I remember that MC barostat,
> at least in case of lipid bilayers, had issues with properly equilibriating
> the structure. If I recall correctly, for instance, that the xy-dimensions
> of the box didn’t change considerably from the initial values and the area
> per lipid values did not converge. I think using Berendsen barostat with
> the semi-isotropic coupling solved most of these problems. After the
> structure is equilibrated , one can switch back to the MC barostat if
> desired.
>
> I’m not sure if this will solve the problem but could be worth giving it a
> try.
>
> Best,
> Batuhan
>
> > On 3 Jun 2020, at 15:17, David A Case <david.case.rutgers.edu> wrote:
> >
> > On Wed, Jun 03, 2020, Dave Cerutti wrote:
> >
> >> If you could provide a link to the archive where that advice is found, I
> >> might be able to comment more, but for now I looked through the manual
> and
> >> found the "baroscalingdir" keyword, which is there to control the
> >> directions along which the box stretches during constant pressure
> >> dynamics. This keyword is applicable with the Monte-Carlo barostat and
> >> anisotropic pressure control. Since semi-isotropic is just a special
> case
> >> of anisotropic, I don't see a reason you cannot combine ntp=3 and
> >> barostat=2. The detailed balance condition on the MC barostat is
> >> important, so someone may have made a note of DtB considerations if the
> box
> >> does not stretch isotropically. However, if baroscalingdir can be an
> >> option, any caveats are either solved or at least one person who
> programmed
> >> the MC barostat thinks it's OK.
> >>
> >>>
> >>> I've also read, back in the Amber forums, that when you use ntp=3, you
> >>> should use barostat=1. Is this true?
> >>>
> >
> > Please check with Prof. Wonpil Im at Lehigh University. I think I
> > remember that he has looked at this problem, but I'm not sure if this
> > has been published. Other Amber users running membrane simulations with
> > Amber force fields and the MC barostat might want to comment on their
> > experience.
> >
> > ....dac
> >
> >
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Received on Thu Jun 04 2020 - 14:30:01 PDT
