You got pretty close to what the density of SPC water is at 298K

According to "Temperature dependence of TIP3P, SPC, and TIP4P water from NPT 
Monte Carlo simulations: Seeking temperatures of maximum density", William L. 
Jorgensen, Corky Jenson, J. Comp. Chem, Volume 19 Issue 10, Pages 1179 - 1186 
(1998)
it should be 0.985g/ml, and TIP4P will give you 1.001g/ml. 

Remember that you're simulating SPC water, not real water. 

Sander



On Feb 24, 2010, at 13:23 , Sunil Thapa wrote:

> Respectable Experts
> Grettings
> I am studying diffusion of an oxygen molecule in 255 molecule of SPC water. I 
> need to determine diffusion coefficient at temperature 298 K and 1 atm 
> pressure. In order to do that
> using editconf I scaled the density of the system including oxygen to 1g/l. I 
> checked the density after energy minimization (steepest descent) which gave 
> the reading of 0.76 g/l. After that I equilibrated the system to pressure of 
> 1 bar and temperature of 298 K using NPT simulation (berendsen). The density 
> reading was 0.98. When I checked the density table at 298 K, it was about 
> 0.9927 g/l.
> I am afraid if my density is not consistent with experimental density of the 
> system. Is this some bug of gromacs 3.2.1 or my understanding of 
> thermodynamics is very poor for pressure and liquid density. Please help me 
> on this.
> Should I use the same NPT simulation to derive the experimental diffusion 
> coefficient ?
>  
> Looking forward to hearing from you
> Neal, Nepal
> 
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