[EXTERNAL] ewald accuracy

Dear Paul,
Many thanks for your email. I am using March-2-2012 version and below is the input script:
I have set the ewald accuracy to 1e-6 but when I go to the ewald.cpp, ewald::compute function and print the accuracy I get the value of 0.00033.
I wonder if you could elaborate on this problem


(1) Initialization

units real
dimension 3
boundary p p p
newton on
atom_style full
#processors 2 2 2

kspace_style ewald 1.0e-6
pair_style lj/cut/coul/long 11.2664
pair_modify table 0

bond_style harmonic
angle_style harmonic

dihedral_style harmonic

improper_style cvff

(2) Atom definition

read_data Prop_atom_True.lammps05

(3) Settings

neighbor 2.0 bin
special_bonds lj/coul 0.0 0.0 1.0

(5) Dynamics

timestep 1.0
velocity all create 300 4928459 rot yes dist uniform
fix 1 all nvt temp 300.0 300.0 100.0
thermo_style custom step etotal ke temp pe ebond eangle edihed eimp evdwl ecoul elong press pxx pyy pzz
thermo 500
restart 500000 Pre_poly.restart

dump 1 all atom 10000 arc_1st.class2
dump_modify 1 image yes scale yes flush yes
dump 2 all dcd 10000 traj_1st.dcd

run 500000
undump 1
undump 2
unfix 1

You’re probably looking at the absolute RMS force accuracy. The 1e-6 value you input specifies the estimated relative force accuracy. The difference is that the latter includes a normalization factor. Both the absolute and relative accuracies are printed to the screen and the logfile during Ewald initialization. You should see something like this:

estimated absolute RMS force accuracy = 0.000332064

estimated relative force accuracy = 1e-06

Please review the documentation for more information:




The specified accuracy determines the relative RMS error in per-atom forces calculated by the long-range solver. It is set as a dimensionless number, relative to the force that two unit point charges (e.g. 2 monovalent ions) exert on each other at a distance of 1 Angstrom. This reference value was chosen as representative of the magnitude of electrostatic forces in atomic systems. Thus an accuracy value of 1.0e-4 means that the RMS error will be a factor of 10000 smaller than the reference force.

The accuracy setting is used in conjunction with the pairwise cutoff to determine the number of K-space vectors for style ewald or the FFT grid size for style pppm.

RMS force errors in real space for ewald and pppm are estimated using equation 18 of (Kolafa), which is also referenced as equation 9 of (Petersen). RMS force errors in K-space for ewald are estimated using equation 11 of (Petersen), which is similar to equation 32 of (Kolafa). RMS force errors in K-space for pppm are estimated using equation 38 of (Deserno).