[gmx-users] setting cut-off for coulomb interaction

[Rui Qiao]

Hi, there:
	I used cut-off for coulomb interaction in my simulation, and when
I am using:
	rcoulomb = 2.4 (my simulation box is 5nm), the energy minimization
give lowest potential energy to be -7.8e4, however, if I change 
	rcoulomb = 0.9, it give a number of -1.15e5. 

	Actually I am expecting it to be around -1.5e5 becuase I am having
3600 water moleculars (using PME to do the simulation yields a number
around -1.1e5 too). What confuse me is why when using a larger cutoff
radii give a "bad" result.

	Another question is that when using plain cutoff, shall I specify
the epsion_r for water? The example problems online seems does not give
such an epsion_r.

	Any comment is welcomed.

	Thanks for your time.

	Rui
	

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Rui Qiao
Research Assistant
Beckman Institute, UIUC