[gmx-users] PME question in GMX4.6.5

[Mark Abraham]

On Sat, Jan 10, 2015 at 5:58 PM, Tom <dnaafm at gmail.com> wrote:

> Dear GMX and PEM experts,
>
> I am using gromacs 4.6.5.  My system is neutral charge (net charge=0).
> I used PME with the following options:
> -------------
> ; Electrostatics
> coulombtype              = PME  ; Particle Mesh Ewald for long-range
> electrostatics
> ewald_geometry           = 3dc
> pme_order                = 4    ; cubic interpolation
> fourierspacing           = 0.12 ; grid spacing for FFT
> --------------
>
> I noticed that for my system *with PME calculation *that value of *Coulomb
> (SR) is much*
> *larger than Coul. recip*
> Coulomb (SR) (= 3.88670e+05) < Coul. recip.(3.09251e+06)
>
> Do you think if it is possible to have such as huge tail effects of
> Coulombic interactions?
>

You chose a set of parameters that split the computation of the full
periodic electrostatic sum into two terms whose sum approximates the full
one while hopefully being cheaper to compute. The relative magnitude of the
energies doesn't mean much of anything...

(My systems consist of a neutral SAMs surface and water)
> Another problem is that g_energy of GMX4.6.5 only reports the value of
> Coulomb (SR) and
> does not report Coul. recip.
>

I'll bet lunch that the input was not actually from the PME run ;-)

I also used simple *cutoff* to calcualte coulombic:
> Coulomb (SR) = 3.99204e+06  *is the very close to the PME *calculation of
> the total of Coulomb (SR) + Coul. recip.
>

Great, but this is a totally different (and terrible) approximation.

Mark


> Thanks a lot for your help!
>
> Thomas
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