# RE: How to get the right density of a methane system

From: Nielsen, Steven (steven.nielsen_at_utdallas.edu)
Date: Fri Aug 11 2017 - 23:38:18 CDT

Hi Monika,

Yes, the problem is the force field. For example, in the following article you can see that the density of water is often incorrect:
http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1096-987X(19980730)19:10%3C1179::AID-JCC6%3E3.0.CO;2-J/abstract

There are no special techniques, you should just set the box size (in NVT) yourself if you want a specified density.
Or, you could change the temperature until you get the desired density, which would show you how bad the force field is...

Worst case, you could get the wrong phase. For example the popular water models SPC, SPC/E, TIP3P and TIP4P produce
poor agreement with water's melting point (giving melting points of 190 K, 215 K, 146 K and 232 K respectively).
I'm quoting here from the web site:
http://www1.lsbu.ac.uk/water/water_models.html

-Steve
________________________________________
Sent: Friday, August 11, 2017 9:56 PM
To: <namd-l_at_ks.uiuc.edu>
Subject: namd-l: How to get the right density of a methane system

Dear all,

I want to simulate a system of liquid methane at 91 K and 1 atm. I used some opls parameters from literature and converted them to CHARMM and used for my simulation. Initially I heated the system in NVT and then performed NPT. The system equillibriates but the density of methane after equillibration is higher than the experimental values. What could be the problem, the force field? or are there any special techniques to get the right density of a system. I am using NAMD with CHARMM for my simulations.

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