Dear AMBER folks,
I have been doing fully atomistic simulations of a protein in a POPC
bilayer using AMBER18 with the AMBER forcefields. The system has
~305,000 atoms and box dimensions of 200x200x80Å.
I have repeatedly encountered a problem where after ~100ns of
unrestrained (production) simulation it becomes apparent that the box is
shrinking in the X and Y dimensions, while growing in the Z dimension.
This behavior is seen for multiple systems of the same size. For
production, I use the following parameters:
A NPT simulation for common production-level simulations
&cntrl
imin=0, ! No minimization
irest=1, ! This IS a restart of an old MD simulation
ntx=5, ! So our inpcrd file has velocities
ig=-1,
! Temperature control
ntt=3, ! Langevin dynamics
gamma_ln=1.0, ! Friction coefficient (ps^-1)
temp0=303.15, ! Target temperature
! Potential energy control
cut=10.0, ! Nonbonded cutoff, in Angstroms
! MD settings
nstlim=100000000, ! 100M steps, 200 ns total
dt=0.002, ! time step (ps)
! SHAKE
ntc=2, ! Constrain bonds containing hydrogen
ntf=2, ! Do not calculate forces of bonds containing hydrogen
tol=0.0000001, ! SHAKE tolerance
! Control how often information is printed
ntpr=100000, ! Print energies every 100K steps
ntwx=100000, ! Print coordinates every 100K steps to the trajectory
ntwr=100000, ! Print a restart file every 100K steps (can be less frequent)
ntxo=2, ! Write NetCDF format
ioutfm=1, ! Write NetCDF format (always do this!)
! Wrap coordinates when printing them to the same unit cell
!iwrap=1,
! Constant pressure control.
barostat=2, ! MC barostat... change to 1 for Berendsen
ntp=3, ! 1=isotropic, 2=anisotropic, 3=semi-isotropic w/ surften
pres0=1.0, ! Target external pressure, in bar
taup=1.0, ! Was 10 in Yessica's mineq but has been reset to default here
! Constant surface tension (needed for semi-isotropic scaling). Uncomment
! for this feature. csurften must be nonzero if ntp=3 above
csurften=3, ! Interfaces in 1=yz plane, 2=xz plane, 3=xy plane
gamma_ten=0.0, ! Surface tension (dyne/cm). 0 gives pure semi-iso scaling
ninterface=2, ! Number of interfaces (2 for bilayer)
ntb=2, ! pbc
/
&wt type='END' / ! End of varying conditions
/
LISTIN=POUT
LISTOUT=POUT
&end
END
However, if I start a parallel simulation with the same parameter files
using the Berendsen barostat instead of MC (only change is barostat=1),
I do NOT observe the box compression even at 200+ ns of run-time. I
believe this also speaks to the question raised by David Case in a
recent thread: http://archive.ambermd.org/202006/0010.html
Why might this be happening, and is there a way I can fix it?
--
Yessica
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Received on Sun Jul 12 2020 - 17:00:03 PDT
