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On 21/08/2011 2:08 PM, Elisabeth wrote:
<blockquote
cite="mid:CAGioFsi4z3=X5o8mPmzD7z-2g9tEsxhA_5Yn9xHN-ZEVMv150Q@mail.gmail.com"
type="cite"><br>
<br>
<div class="gmail_quote">On 20 August 2011 21:20, Mark Abraham <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:Mark.Abraham@anu.edu.au">Mark.Abraham@anu.edu.au</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div bgcolor="#FFFFFF" text="#000000">
<div class="im"> On 20/08/2011 8:02 AM, Elisabeth wrote:
<blockquote type="cite">Dear experts,<br>
<br>
I am intending to calculate the equilibrium density of a
pure hydrocarbon at different pressures , at 425 K. The
normal boiling point of the liquid is around 350 K. For
pressures below 100 bar densities I am getting from NPT
is in accurate. I start form a structure which is
compressed to above 0.6 g/cm3 density but since
temperature is high density goes than to the values
below. <br>
<br>
P = 50 bar > NPT rho= 0.344 experimental density
~ 0.54<br>
P 100 > NPT rho= 0.43 experimental
density ~ 0.55<br>
<br>
densities become more accurate for P >100 bar. <br>
<br>
P 500 > rho= 0.56 experimental density ~
0.61<br>
<br>
I thought maybe you have some idea on how this
inaccuracy can be improved.<br>
</blockquote>
<br>
</div>
These could reflect limitations in the model you are using
(which almost certainly wasn't parametrized upon data like
this). Your integration time step is twice as large as is
commonly used in the absence of constraints. Also, be sure
you are measuring your density only after equilibration, not
an average that includes the equilibration period - and
describe that method so people here know you're making such
sensible measurements.<br>
</div>
</blockquote>
<div><br>
Hello Mark,<br>
<br>
Thank you for your comment. Densities are equilibrated. I am
not sure what you mean by limitations in the model.</div>
</div>
</blockquote>
<br>
Parameters are not magic truth values. They're empirical values that
have been observed to replicate some experimental or computational
results - almost invariably room temperature and atmospheric
pressure. It is not necessarily known how well they reproduce other
conditions - increasing temperature and pressure may well lead to
those parameters modelling some alternative reality. Taking a small
test system and systematically trialling small increases in
temperature and/or pressure from "normal" will tend to reveal such
trends. Such results may indicate the need for different
parametrization. This is typically expensive in computer and human
time.<br>
<br>
Mark<br>
<br>
<blockquote
cite="mid:CAGioFsi4z3=X5o8mPmzD7z-2g9tEsxhA_5Yn9xHN-ZEVMv150Q@mail.gmail.com"
type="cite">
<div class="gmail_quote">
<div> I actually fixed the density and tried NVT using the
experimental density at 298 and found a full agreement between
vaporization heat of the liquid and that reported in the
original OPLS paper. Once I try to obtain the density using
NPT this inaccuracy in densities appear. I have been using 1
fs dt with berendsen barostat for 5 ns and collected data over
the last 1 ns ( there are about 2500 atoms in the system). So
now I am going to follow your instruction and try dt of 0.5 fs
for another 5ns along with cpt files from previous barostat
trial (berendsen). I will get back to you to report the
results soon.<br>
<br>
BTW: I am using version 4.5.4<br>
<br>
Thank you,<br>
<br>
<br>
</div>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
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<font color="#888888"> <br>
Mark</font>
<div>
<div class="h5"><br>
<br>
<blockquote type="cite">Thank you in advance for your
invaluable help.<br>
Best,<br>
<br>
<br>
; Bonds<br>
constraints = none <br>
constraint-algorithm = lincs<br>
<br>
; Run control <br>
integrator = md <br>
dt = 0.001 <br>
nsteps = 5000000 <br>
nstcomm = 100 <br>
<br>
; Output control<br>
nstenergy = 100 <br>
nstxout = 100 <br>
nstvout = 0<br>
nstfout = 0<br>
nstlog = 1000 <br>
nstxtcout = 1000 <br>
<br>
; Neighbor searching<br>
nstlist = 10 <br>
ns_type = grid <br>
<br>
; Electrostatics/VdW <br>
coulombtype = Shift <br>
vdw-type = Shift <br>
rcoulomb-switch = 0 <br>
rvdw-switch = 0.9 <br>
<br>
; Cut-offs<br>
rlist = 1.25 <br>
rcoulomb = 1.0 <br>
rvdw = 1.0 <br>
<br>
; Temperature coupling <br>
Tcoupl = v-rescale <br>
tc-grps = System <br>
tau_t = 0.1 <br>
ref_t = 425 <br>
<br>
; Pressure coupling<br>
Pcoupl = berendsen <br>
Pcoupltype = isotropic <br>
tau_p = 1 <br>
compressibility = 3.5e-5 <br>
ref_p = 100 <br>
<br>
; Velocity generation <br>
gen_vel = no;yes <br>
gen_temp = 425 <br>
gen_seed = 173529 <br>
<br>
<br>
<br>
<br>
<br>
<fieldset></fieldset>
<br>
</blockquote>
<br>
</div>
</div>
</div>
<br>
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