On 15/02/2012 2:46 AM, Qiao Baofu wrote:
Hi all,
Anybody can help me out! Thanks in advance?
In my test simulation, there are one Na^+ and one Cl^- (the distance
of 1nm) in vacuum in a very big simulation box (10*10*10 nm^3). I
calculated the energies under two different conditions,
"coulombtype=PME" vs "coulombtype=cut-off".
1. "coulombtype=PME"
Coul. recip. -139.191 -- 0 0
(kJ/mol)
Coul-SR:NA-CL *-0.0322805* -- 0 0 (kJ/mol)
LJ-SR:NA-CL -0.00100285 -- 0 0
(kJ/mol)
2. "coulombtype=cut-off"
Coul-SR:NA-CL *-138.935* -- 0 0 (kJ/mol)
LJ-SR:NA-CL -0.00100285 -- 0 0
(kJ/mol)
I did some calculations. Under "coulombtype=cut-off", E(Coul-SR) =
f*e_i*e_j/r with f=138.935, which is what I expect.
However under "coulombtype=PME", how does Gromacs get the value of
-0.0322805kJ/mol for "*SR*" term? If I understand it correctly, "SR"
means "short-range", which is within the cut-off distance. Since the
cut-off distance (1.2nm here) is much smaller than the simulation box
length (10nm), there is not short-range interaction with the image box.
Your two atoms are within the cut-off distance, so their interaction
contributes to Coul-SR:NA-CL. The total energy for that interaction is
distributed over the Coul-SR and Coul. recip terms (which is how PME
works...), which you will note is approximately equal to Coul-SR for
coulombtype cut-off.
By construction, your periodic images are too far away to contribute to
this term in either case (and you'd have been prevented by grompp from
getting this far, if that were not true).
Some details:
Gromacs 4.5.5
PBC is used.
distance of Na^+ and Cl^-: 1nm
Na^+ and Cl^- are frozen in XYZ dimensions.
Only 1 step is run.
Using mdrun -rerun is often the best way to calculate single-point
energies/forces.
Mark
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