change neighbor list defaults to delay=0, every=1, check=yes

doc/src/neigh_modify.rst

@@ -16,12 +16,12 @@ Syntax
 
      keyword = *delay* or *every* or *check* or *once* or *cluster* or *include* or *exclude* or *page* or *one* or *binsize* or *collection/type* or *collection/interval*
        *delay* value = N
-         N = delay building until this many steps since last build
+         N = delay building neighbor lists until this many steps since last build
        *every* value = M
-         M = build neighbor list every this many steps
+         M = consider building neighbor lists every this many steps
        *check* value = *yes* or *no*
-         *yes* = only build if some atom has moved half the skin distance or more
-         *no* = always build on 1st step that *every* and *delay* are satisfied
+         *yes* = only build if at least one atom has moved half the skin distance or more
+         *no* = always build on 1st step where *every* and *delay* are conditions are satisfied
        *once* value = *yes* or *no*
          *yes* = only build neighbor list once at start of run and never rebuild
          *no* = rebuild neighbor list according to other settings
@@ -71,30 +71,48 @@ Description
 """""""""""
 
 This command sets parameters that affect the building and use of
-pairwise neighbor lists.  Depending on what pair interactions and
-other commands are defined, a simulation may require one or more
-neighbor lists.
-
-The *every*, *delay*, *check*, and *once* options affect how often
-lists are built as a simulation runs.  The *delay* setting means never
-build new lists until at least N steps after the previous build.  The
-*every* setting means build lists every M steps (after the delay has
+pairwise neighbor lists.  Depending on what pair interactions and other
+commands are defined, a simulation may require one or more neighbor
+lists.
+
+The *every*, *delay*, *check*, and *once* options affect how often lists
+are built as a simulation runs.  The *delay* setting means never build
+new lists until at least N steps after the previous build.  The *every*
+setting means attempt to build lists every M steps (after the delay has
 passed).  If the *check* setting is *no*, the lists are built on the
 first step that satisfies the *delay* and *every* settings.  If the
 *check* setting is *yes*, then the *every* and *delay* settings
 determine when a build may possibly be performed, but an actual build
-only occurs if some atom has moved more than half the skin distance
-(specified in the :doc:`neighbor <neighbor>` command) since the last
-build.
-
-If the *once* setting is yes, then the neighbor list is only built
-once at the beginning of each run, and never rebuilt, except on steps
-when a restart file is written, or steps when a fix forces a rebuild
-to occur (e.g. fixes that create or delete atoms, such as :doc:`fix deposit <fix_deposit>` or :doc:`fix evaporate <fix_evaporate>`).
-This setting should only be made if you are certain atoms will not
-move far enough that the neighbor list should be rebuilt, e.g. running
-a simulation of a cold crystal.  Note that it is not that expensive to
-check if neighbor lists should be rebuilt.
+only occurs if at least one atom has moved more than half the neighbor
+skin distance (specified in the :doc:`neighbor <neighbor>` command)
+since the last neighbor list build.
+
+.. admonition:: Impact of neighbor list settings
+   :class: note
+
+   The choice of neighbor list settings can have a significant impact on
+   the (parallel) performance of LAMMPS and the correctness of the
+   results.  Since the building of neighbor lists is time consuming,
+   doing it less frequently can speed up a calculation.  However, if the
+   lists are rebuilt too infrequent, interacting pairs are missed and
+   thus the resulting pairwise interactions incorrect.  The optimal
+   settings depend on the properties of the simulated system (like
+   density, geometry, temperature, pressure), the force field settings,
+   the size of the timestep, neighbor list skin distance and so on.  The
+   default settings are chosen to be very conservative to promote
+   correctness of the simulation and they depend on the *check* flag
+   heuristics to reduce the number of neighbor list rebuilds at a minor
+   expense for executing the check.
+
+If the *once* setting is yes, then the neighbor list is only built once
+at the beginning of each run, and never rebuilt, except on steps when a
+restart file is written, or steps when a fix forces a rebuild to occur
+(e.g. fixes that create or delete atoms, such as :doc:`fix deposit
+<fix_deposit>` or :doc:`fix evaporate <fix_evaporate>`).  This setting
+should only be made if you are certain atoms will not move far enough
+that the neighbor list should be rebuilt, e.g. running a simulation of a
+cold crystal.  Note that it is not that expensive to check if neighbor
+lists should be rebuilt.
 
 When the rRESPA integrator is used (see the :doc:`run_style <run_style>`
 command), the *every* and *delay* parameters refer to the longest
@@ -234,11 +252,11 @@ depend on their atom type.
 Restrictions
 """"""""""""
 
-If the "delay" setting is non-zero, then it must be a multiple of the
-"every" setting.
+If the *delay* setting is non-zero, then it must be a multiple of the
+*every* setting.
 
-The molecule/intra and molecule/inter exclude options can only be used
-with atom styles that define molecule IDs.
+The *molecule/intra* and *molecule/inter* exclusion options can only
+be used with atom styles that define molecule IDs.
 
 The value of the *page* setting must be at least 10x larger than the
 *one* setting.  This insures neighbor pages are not mostly empty
@@ -252,6 +270,6 @@ Related commands
 Default
 """""""
 
-The option defaults are delay = 10, every = 1, check = yes, once = no,
+The option defaults are delay = 0, every = 1, check = yes, once = no,
 cluster = no, include = all (same as no include option defined),
 exclude = none, page = 100000, one = 2000, and binsize = 0.0.

src/neighbor.cpp

@@ -119,7 +119,7 @@ pairclass(nullptr), pairnames(nullptr), pairmasks(nullptr)
 
   style = Neighbor::BIN;
   every = 1;
-  delay = 10;
+  delay = 0;
   dist_check = 1;
   pgsize = 100000;
   oneatom = 2000;