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On 20/08/2011 8:02 AM, Elisabeth wrote:
<blockquote
cite="mid:CAGioFsiCbR26UvOuXFV5fbpLopcFwNNiHi3rQ3zc_DuVd=PeFw@mail.gmail.com"
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>
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>
<br>
Mark<br>
<br>
<blockquote
cite="mid:CAGioFsiCbR26UvOuXFV5fbpLopcFwNNiHi3rQ3zc_DuVd=PeFw@mail.gmail.com"
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>
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<br>
</blockquote>
<br>
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