# [gmx-users] Reduced Units

pojeda at icp.uni-stuttgart.de pojeda at icp.uni-stuttgart.de
Mon Aug 9 15:58:43 CEST 2010

Hi,

I want to use the pme algorithm in GROMACS. Thge size of my box is
8.2x8.2x8.2 in reduced units. I am considering a collection of 1000
of CO2 molecules. If I am using reduced units, the values of the
parameters "rcoulomb=0.9" I think should be converted to reduced
units "rcoulomb=2.45" (my \sigma=0.366nm). But my question is how
can one convert the value of "fourierspacing=0.12" to reduced units?

(This script is in SI standard units)
; OPTIONS FOR ELECTROSTATICS AND VDW
; Method for doing electrostatics
coulombtype              = pme
rcoulomb-switch          = 0
rcoulomb                 = 0.9
; Dielectric constant (DC) for cut-off or DC of reaction field
epsilon-r                = 1
; Method for doing Van der Waals
vdw-type                 = Cut-off
; cut-off lengths
rvdw-switch              = 0
rvdw                     = 0.9
; Apply long range dispersion corrections for Energy and Pressure
DispCorr                 = EnerPres
; Extension of the potential lookup tables beyond the cut-off
table-extension          = 1
; Spacing for the PME/PPPM FFT grid
fourierspacing           = 0.12
; FFT grid size, when a value is 0 fourierspacing will be used
fourier_nx               = 0
fourier_ny               = 0
fourier_nz               = 0
; EWALD/PME/PPPM parameters
pme_order                = 4
ewald_rtol               = 1e-05
ewald_geometry           = 3d
epsilon_surface          = 0
optimize_fft             = no

Thanks.