I turned off the torsion interaction. The difference between Lammps and
Gromacs at integration time step 2fs was reduced. Details below:
A melt of 240 n-octane (united-atom model), NVT, T=300K, V=55.46nm^3.
Both Lammps and Gromacs use berendsen thermostat with tau_t=1ps.
Integration time step 1fs:
Lammps Gromacs Std. Err. (for both)
Ebond(kJ/mol): 2092 2109 100
Eangle: 1778 1754 80
Elj+corr: -10501 -10553 100
T(K): 300 299 5
P(atm): 3188 3016 700
integration time step 2fs:
Lammps Gromacs Std.Err. (for both)
Ebond: 2133 2232 100
Eangle: 1803 1737 80
Elj+corr: -10501 -10623 100
T: 300 298 5
P: 3133 2955 600
Lammps results remain almost unchanged when dt increases from 1fs to 2fs,
and Ebond : Eangle = 7 : 6, which is the ratio between # of bonds and
# of angles.
Gromacs results change more significantly when dt goes from 1fs to 2fs.
and the trend of Ebond and Eangle are opposite. It is more significant
when torsion interaction is present.
On Tue, 3 Nov 2009, David van der Spoel wrote:
Peng Yi wrote:
Hi, David,
I used Berendsen thermostat and a bigger tau_t=1ps to redo the simulations.
The general conclusion is the same. The std err is the same in both
packages. And during each simulation, the integration time does not
change. Details below:
Integration time step 1fs:
Lammps Gromacs Std.Err. (for both)
Ebond(kJ/mol): 2112 2170 100
Eangle: 1799 1770 100
Etors: 2552 2490 100
Elj+corr: -10711 -10777 100
P(atm): 3250 3216 500
Integration time step 2fs:
Lammps Gromacs Std.Err. (for both)
Ebond: 2154 2654 120
Eangle: 1840 1645 120
Etors: 2573 2236 120
Elj+corr: -10711 -11019 100 P(atm): 3250
2590 600
How about the temperature in both systems? Was Lammps also run with
Berendsen? It could also still be a topology error. Maybe you can turn off
the torsion potential to test this.
-Peng
On Sun, 1 Nov 2009, David van der Spoel wrote:
Peng Yi wrote:
Thank for your reply! I have done some NVT runs per your suggestion, and
the results are similar to NPT runs, i.e., Gromacs results is more
affected by changing integration timestep than Lammps. Details below:
A melt of 240 octane chains by united-atom model. T=300K, V=55.46 nm^3.
Both Gromacs and Lammps use Nose-Hoover thermostat with tau_t=0.2 ps.
As some people have note the tau_t is short for Nose Hoover. Are you sure
this means the same in Lammps and in Gromacs? For one thing, there is no
tau_t in the NH algorithm as far as I know, and Gromacs converts it to an
appropriate weight or whatever that is called. What does Lammps do with
this tau_t. To be a the safe side you could run both with Berendsen as
well. Is the std err identical in both packages? And in the 2 fs run, are
both simulation equlibrated with this time step as well?
All other parameters in .top and .mdp files are the same as previously
attached..
If I use integration time 1fs, Lammps and Gromacs produce
consistent results:
Lammps Gromacs std. err.
Ebond(kJ/mol): 2133 2160 100
Eangle: 1757 1780 80
Etors: 2531 2510 80
Elj+corr: -10711 -10767 90
P(atm): 3500 3250 500
if I use integration time 2fs, Lammps results remain unchanged, but
Gromacs results change significantly, particularly bonded energy:
Lammps Gromacs std. err.
Ebond(kJ/mol): 2175 2710 100
Eangle: 1799 1640 70
Etors: 2573 2230 80
Elj+corr: -10711 -11007 100 P(atm): 3200
2730 700
Would that be a result of using different integrator between Lammps and
Gromacs? Lammps uses Velocity-Verlet, and Gromacs uses Leap-frog.
Thanks,
-Peng
On Thu, 29 Oct 2009, David van der Spoel wrote:
aherz wrote:
Hey,
are you running single or double precision gromacs?
Afaik, depending on the circumstances the energy drift in gromacs can
be
rather bad for single precision.
Please refer to the gromacs 4.0 paper for a discussion of the drift.
If you want to compare energies you need the same density, which you do
not have, you may need to run NVT for that.
Note that your integration time step is quite large, and the temperature
coupling constant is very small.
You could try a shifted LJ + dispersion correction, it is not clear to
me how LAMMPS treats cutoffs, couldn't find it in the manual.
Alex
Peng Yi schrieb:
On Wed, 28 Oct 2009, Mark Abraham wrote:
Peng Yi wrote:
I am trying to simulate alkane melt and found out that gromacs and
lammps gave different results, particularly the bonded interaction
energy.
I wonder if anyone has such experience. Thanks,
Even two installations of the same version of GROMACS can give
different results. The question is whether when using comparable
model physics you observe the same ensemble averages.
Mark
Hi, Mark,
Thanks for reply! The difference is statistically significant. And I
am
wondering if it is caused by the integrator: Leap-frog for Gromacs and
Velocity-verlet for Lammps. Detail description of the comparison
please
see below:
It is an NPT simulation of a melt of 240 n-octane molecules using
united-atom model, i.e., CHx group is considered as one atom. There
are
bond, angle, torsion and LJ interactions. T=300K and P=1atm.
Lammps uses nose-hoover thermostat and barostat, and Gromacs uses
nose-hoover thermostat and Parranello-Rahman barostat. Time constants
for
thermostat and barostat are 0.02ps and 2.0ps, respectively.
If I use integration time 1fs, Lammps and Gromacs gave consistent
results:
Lammps Gromacs
Ebond(kJ/mol): 2092 2146
Eangle: 1757 1760
Etors: 2510 2500
Elj+corr: -9238 -9350
Volume(nm^3): 66.7 66.5
where energy fluctuation is 100 kJ/mol and volume fluctuation is 1
nm^3,
Elj+corr is the total LJ energy including tail correction.
However, if I use integration time 2fs, Lammps results do not change
much, but Gromacs results changed a lot:
Lammps Gromacs
Ebond(kJ/mol): 2133 2700 Eangle: 1799
1640
Etors: 2552 2200
Elj+corr: -9292 -9886 Volume: 66.7
64.0
The results given by Lammps is more reasonable because the Ebond
should
be equal to the total # of bonds times 1/2k_BT and Eangle should be
equal
to the total # of angles times 1/2k_BT. At T=300K, 1/2k_BT=1.25
kJ/mol.
240 n-octanes have total 1680 bonds and 1440 angles.
The bond and angle interactions are both harmonic functions. Bond
interaction constant kl=292880 kJ/mol/nm^2, corresponding to a bond
ossilation period 16 fs.
Is there something related to the integrator?
Here I attached my grompp.mdp and topol.top files.
##########
grompp.mdp
##########
; VARIOUS PREPROCESSING OPTIONS
title = Yo
cpp = /usr/bin/cpp
include = define =
; RUN CONTROL PARAMETERS
integrator = md
tinit = 0
dt = 0.001
nsteps = 2000000
init_step = 0
comm-mode = Linear
nstcomm = 1
comm-grps =
; OUTPUT CONTROL OPTIONS
nstxout = 5000
nstvout = 5000
nstfout = 5000
nstcheckpoint = 10000
nstlog = 1000
nstenergy = 1000
nstxtcout = 5000
xtc-precision = 1000
xtc-grps = energygrps =
; NEIGHBORSEARCHING PARAMETERS
nstlist = 10
ns_type = grid
pbc = xyz
rlist = 1.0025
domain-decomposition = no
; OPTIONS FOR ELECTROSTATICS AND VDW
coulombtype = Cut-off
rcoulomb-switch = 0
rcoulomb = 1.0025
epsilon-r = 1
vdw-type = Cut-off
rvdw-switch = 0 ; default rvdw
= 1.0025 ; default 1 nm
DispCorr = EnerPres
;table-extension = 1.5
fourierspacing = 0.12
fourier_nx = 0
fourier_ny = 0
fourier_nz = 0
pme_order = 4
ewald_rtol = 1e-05
ewald_geometry = 3d
epsilon_surface = 0
optimize_fft = no
; OPTIONS FOR WEAK COUPLING ALGORITHMS
Tcoupl = nose-hoover
tc-grps = System
tau_t = 0.02
ref_t = 300.0
Pcoupl = Parrinello-Rahman
Pcoupltype = isotropic
tau_p = 2.0
compressibility = 4.5e-5
ref_p = 1.0
andersen_seed = 815131
; GENERATE VELOCITIES FOR STARTUP RUN
gen_vel = yes
gen_temp = 300
gen_seed = 2009
; OPTIONS FOR BONDS constraints = none
constraint-algorithm = Lincs
unconstrained-start = no
Shake-SOR = no
shake-tol = 1e-04
lincs-order = 4
lincs-iter = 1
lincs-warnangle = 30
morse = no
; ENERGY GROUP EXCLUSIONS
; Pairs of energy groups for which all non-bonded interactions are
excluded
energygrp_excl =
; NMR refinement stuff disre = No
disre-weighting = Conservative
disre-mixed = no
disre-fc = 1000
disre-tau = 0
nstdisreout = 100
orire = no
orire-fc = 0
orire-tau = 0
orire-fitgrp = nstorireout = 100
dihre = No
dihre-fc = 1000
dihre-tau = 0
nstdihreout = 100
#########
topol.top
#########
#include "ffG53a6.itp"
[atom-types]
;name mass charge ptype V/c6 W/c12
CH2 14.0 0.00 A 0.0 0.0
CH3 15.0 0.00 A 0.0 0.0
[nonbond-params]
; i j func V/c6 W/c12
CH2 CH2 1 0.0078 3.24e-5
CH2 CH3 1 0.0078 3.24e-5
CH3 CH3 1 0.0078 3.24e-5
[ moleculetype ]
; name nrexcl
Octane1 3
[ atoms ]
; nr type resnr residu atom cgnr charge
1 CH3 1 C8 CH3 1 0.0
2 CH2 1 C8 CH2 2 0.0
3 CH2 1 C8 CH2 3 0.0
4 CH2 1 C8 CH2 4 0.0
5 CH2 1 C8 CH2 5 0.0
6 CH2 1 C8 CH2 6 0.0
7 CH2 1 C8 CH2 7 0.0
8 CH3 1 C8 CH3 8 0.0
[ bonds ]
; ai aj funct c0(nm) c1(kJ/mol/nm^2)
1 2 1 0.153 292880.0
2 3 1 0.153 292880.0
3 4 1 0.153 292880.0
4 5 1 0.153 292880.0
5 6 1 0.153 292880.0
6 7 1 0.153 292880.0
7 8 1 0.153 292880.0
[ pairs ]
; ai aj funct c0 c1
; 1 4 1 0.000000e+00 0.000000e+00 ; 2 5 1
0.000000e+00 0.000000e+00 ; 3 6 1 0.000000e+00 0.000000e+00
; 4 7 1 0.000000e+00 0.000000e+00 ; 5 8 1
0.000000e+00 0.000000e+00
[ angles ]
; ai aj ak funct c0(degree)
c1(kJ/mol/rad^-2)
1 2 3 1 109.5 502.08
2 3 4 1 109.5 502.08
3 4 5 1 109.5 502.08
4 5 6 1 109.5 502.08
5 6 7 1 109.5 502.08
6 7 8 1 109.5 502.08
[ dihedrals ]
; ai aj ak al funct c0 c1 c2 c3 c4
c5
1 2 3 4 3 6.4977 16.9868 3.6275 -27.112 0
0
2 3 4 5 3 6.4977 16.9868 3.6275 -27.112 0
0
3 4 5 6 3 6.4977 16.9868 3.6275 -27.112 0
0
4 5 6 7 3 6.4977 16.9868 3.6275 -27.112 0
0
5 6 7 8 3 6.4977 16.9868 3.6275 -27.112 0
0
[ system ]
octane melt
[ molecules ]
Octane1 240
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________________________________________________________________________
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Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596, 75124 Uppsala, Sweden
phone: 46 18 471 4205 fax: 46 18 511 755
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Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone: +46184714205. Fax: +4618511755.
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