[gmx-users] confusion about rlist continues in 3.2

Michael Brunsteiner mbx0009 at yahoo.com
Thu Jan 29 14:57:01 CET 2004

dear gmx-users,

in gromcas-3.2 source (kernel/readir.c) it says:

  /* Allow rlist>rcoulomb for tabulated long range stuff. This just
   * means the interaction is zero outside rcoulomb, but it helps to
   * provide accurate energy conservation.

sounds reasonble to me (this was missing in 3.1.4), however
further down in readir.c (as shown below) we find that rcoulomb 
is again required to be larger than rlist, if one uses cutoff (and 
not shift) ... why ?
doesn't make sense to me ... sounds as if the cutoff for the
electrostatic interactions (rcoulomb) is now greater than
the cut-off used for creating the neighbor-list (rlist) ...
In previous mails in this list it was said that the cut-off
used for making the neighbour-list is actually larger than
rlist but I don't see this "buffer-distance" anywhere in the 
code ...

any clarification appreciated!

the code:

  if ((ir->coulombtype == eelSHIFT) || (ir->coulombtype == eelSWITCH))
    sprintf(err_buf,"With coulombtype = %s rcoulomb_switch must be <
    CHECK(ir->rcoulomb_switch >= ir->rcoulomb);
    if (ir->rcoulomb_switch > ir->rcoulomb-0.0999) { 
      sprintf(warn_buf,"rcoulomb should be 0.1 to 0.3 nm larger than
rcoulomb_switch to account for diffusion and the size of charge
  } else if(ir->coulombtype == eelCUT || ir->coulombtype == eelRF) {
    sprintf(err_buf,"With coulombtype = %s, rcoulomb must be >=
    CHECK(ir->rlist > ir->rcoulomb);

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