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The problem with Ewald sums is that, although it has other great virtues,
it is not strictly conservative, and with the approximations of PME that
becomes a serious problem and pretty well precludes the use of NVE
boundary conditions (at least that is our experience). We
considered some of this using just pure water as a test system
in:<br><br>
<font face="MS Serif, Geneva">Chiu, S.-W., Clark, M., Subramaniam, S.,
and E. Jakobsson. 2000. Collective motion artefacts arising in
long-duration molecular dynamics simulations. <i>J. Comp.
Chem. </i>21:121-131<br><br>
</font>Eric <br><br>
At 01:49 PM 3/4/2003 -0500, you wrote:<br>
<blockquote type=cite class=cite cite>Hi, I am trying to simulate a
protein in flexible spc water and I believe I have very conservative
parameters. I however see a drift in the total energy, and although it's
not too large it will become large as I need the simulation to be several
ns long. If you plot the total energy you see that the system is
consistently heating up. I have equilibrated the system for more than 100
ps doing npt and before that using position restraints,<br>
so everything should be fine ...<br><br>
time step is 1 fs , 1nm for rvdw, fourierspacing is 0.12 for pme ,
i update the neighbor list every step nstlist =1, etc ...<br>
the mdp file is attached.<br>
Do you think it's the PME? if so, what should i use for fourier
spacing and tolerance to make it not drift? I am running single precision
though.<br>
Thanks!!!<br>
Claudio.<br><br>
here is the output from g_energy statistics over 380 ps<br><br>
Energy
Average RMSD
Fluct. Drift Tot-Drift<br>
-------------------------------------------------------------------------------<br>
Bond
15717.5 254.829 252.943
-0.282489 -107.205<br>
Angle
10366 194.383 193.899
0.125137 47.4897<br>
Proper
Dih.
122.033 12.9951 12.9539
0.00944353 3.58383<br>
Ryckaert-Bell.
1424.82 52.2058 52.0629 -0.0352289
-13.3694<br>
LJ-14
1983.58 41.7575 41.6305
-0.0297046 -11.2729<br>
Coulomb-14
11277.9 55.3076 53.6154
0.123927 47.0305<br>
LJ
(SR)
23216.1 415.996 415.932
-0.0663132 -25.1659<br>
Disper.
corr.
-776.671
0
0
0 0<br>
Coulomb
(SR)
-161371 627.813 626.229
0.406744 154.36<br>
Coulomb
(LR)
-36995 51.9416 50.1115
-0.124746 -47.3413<br>
Potential
-135035 251.587 251.203
0.126766 48.1079<br>
Kinetic
En.
37008.7 245.956 245.244
0.170733 64.7933<br>
Total
Energy
-98026.1 35.0014 12.7625
0.297498 112.901<br><br>
<br>
-- <br>
-----------------------------------------------<br>
Claudio Javier Margulis,
Ph.D.
<br>
Chemistry
Department
<br>
Columbia
University
<br>
Web
<a href="http://www.chem.columbia.edu/~claudiom/%A0%A0%A0%A0%A0%A0%A0" eudora="autourl">http://www.chem.columbia.edu/~claudiom/
</a> <br>
Work phone (212)
854-8470
<br>
-----------------------------------------------<br><br>
<br><br>
;;here no constraints and<br>
; nslist=1 nstcomm removed as parameter<br>
;notice that by default gromacs makes center of mass velocity
removal<br>
; here we are trying flexible spc to see if we get better energy
consevation<br>
;<x-tab> </x-tab>User spoel
(236)<br>
;<x-tab> </x-tab>Wed Nov 3
17:12:44 1993<br>
;<x-tab> </x-tab>Input
file<br>
;<br>
title
= Yo<br>
cpp
= /lib/cpp<br>
define
= -DFLEX_SPC<br>
constraints =
none<br>
integrator =
md<br>
dt
= 0.001<x-tab> </x-tab>; ps !<br>
nsteps
= 50000000<x-tab> </x-tab>; 50 ns<br>
;nstcomm
= 1 should not be there?<br>
nstxout
= 50000<br>
nstxtcout
= 1500<br>
nstvout
= 50000<br>
nstfout
= 0<br>
nstlog
= 5000<br>
nstenergy
= 1500<br>
nstlist
= 1<br>
ns_type
= grid ;n-list parameter<br>
rlist
= 1.0 ;cutoff for the short range neighbor list<br><br>
<br>
; OPTIONS FOR ELECTROSTATICS AND VDW = <br>
; Method for doing electrostatics = <br>
coulombtype
= PME<br>
; Method for doing Van der Waals = <br>
vdw-type
= Cut-off<br>
; cut-off lengths = <br>
rvdw-switch
= 0<br>
rvdw
= 1.0<br>
; Apply long range dispersion corrections for Energy and Pressure =
<br>
DispCorr
= EnerPres<br>
; Spacing for the PME/PPPM FFT grid = <br>
fourierspacing
= 0.12<br>
; FFT grid size, when a value is 0 fourierspacing will be used = <br>
fourier_nx
= 0<br>
fourier_ny
= 0<br>
fourier_nz
= 0<br>
; EWALD/PME/PPPM parameters = <br>
pme_order
= 4<br>
ewald_rtol
= 1e-05<br>
ewald_geometry
= 3d<br>
epsilon_surface =
0<br>
optimize_fft
= yes<br>
; Nose-Hoover temperature coupling is off<br>
Tcoupl
= no<br>
tc-grps<x-tab> </x-tab>
=<br>
tau_t
=<br>
ref_t
=<br>
Energy monitoring<br>
energygrps<x-tab> </x-tab>
= Protein SOL Na<br>
; Pressure coupling is off<br>
Pcoupl
= no<br>
Pcoupltype = <br>
tau_p
= <br>
compressibility = <br>
ref_p
= <br>
; Generate velocites is on at 300 K.<br>
gen_vel
= yes<br>
gen_temp
= 300.0<br>
gen_seed
= 173529</blockquote>
<x-sigsep><p></x-sigsep>
---------------------------------<br>
Eric Jakobsson, Ph.D.<br>
Professor, Department of Molecular and Integrative Physiology, and of
Biochemistry<br>
Senior Research Scientist, National Center for Supercomputing
Applications<br>
Professor, Beckman Institute for Advanced Science and Technology<br>
4021 Beckman Institute, mc251<br>
405 N. Mathews Avenue<br>
University of Illinois, Urbana, IL 61801<br>
ph. 217-244-2896 fax
217-244-2909<br><br>
<br>
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