From b1dd616a2f7fcd87b6ba7dc2c7bf03cefe40aca1 Mon Sep 17 00:00:00 2001
From: julient31 <julien.tranchida1@gmail.com>
Date: Wed, 21 Apr 2021 07:43:00 -0600
Subject: [PATCH] correcting a small typo in the checks, and adding a note in
 the documentation of fix precession/spin about the error if a (0,0,0) vector
 is given as an input.

---
 doc/src/fix_precession_spin.rst  | 12 +++++++++++-
 src/SPIN/fix_precession_spin.cpp | 27 ++++++++++++++++++---------
 2 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/doc/src/fix_precession_spin.rst b/doc/src/fix_precession_spin.rst
index 2d6d8e2f7b..183753db5f 100644
--- a/doc/src/fix_precession_spin.rst
+++ b/doc/src/fix_precession_spin.rst
@@ -127,7 +127,7 @@ details on cubic anisotropies.
 
 Style *stt* is used to simulate the interaction between the spins and 
 a spin-transfer torque.
-See equation(7) of :ref:`(Chirac) <Chirac1>` for more details about the 
+See equation (7) of :ref:`(Chirac) <Chirac1>` for more details about the 
 implemented spin-transfer torque term. 
 
 In all cases, the choice of :math:`(x y z)` only imposes the vector
@@ -137,6 +137,16 @@ normalized).
 
 Those styles can be combined within one single command line.
 
+.. note::
+
+   The norm of all vectors defined with the precession/spin command 
+   have to be non-zero. For example, defining
+   "fix 1 all precession/spin zeeman 0.1 0.0 0.0 0.0" would result
+   in an error message. 
+   Since those vector components are used to compute the inverse of the
+   field (or anisotropy) vector norm, setting a zero-vector would result 
+   in a division by zero.
+
 ----------
 
 Restart, fix_modify, output, run start/stop, minimize info
diff --git a/src/SPIN/fix_precession_spin.cpp b/src/SPIN/fix_precession_spin.cpp
index 01df9d3750..7cfbe6e1a7 100644
--- a/src/SPIN/fix_precession_spin.cpp
+++ b/src/SPIN/fix_precession_spin.cpp
@@ -143,7 +143,8 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
   double norm2,inorm;
   if (zeeman_flag) {
     norm2 = nhx*nhx + nhy*nhy + nhz*nhz;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nhx *= inorm;
     nhy *= inorm;
@@ -152,7 +153,8 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
 
   if (stt_flag) {
     norm2 = nsttx*nsttx + nstty*nstty + nsttz*nsttz;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nsttx *= inorm;
     nstty *= inorm;
@@ -161,7 +163,8 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
 
   if (aniso_flag) {
     norm2 = nax*nax + nay*nay + naz*naz;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nax *= inorm;
     nay *= inorm;
@@ -170,21 +173,24 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
 
   if (cubic_flag) {
     norm2 = nc1x*nc1x + nc1y*nc1y + nc1z*nc1z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nc1x *= inorm;
     nc1y *= inorm;
     nc1z *= inorm;
     
     norm2 = nc2x*nc2x + nc2y*nc2y + nc2z*nc2z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nc2x *= inorm;
     nc2y *= inorm;
     nc2z *= inorm;
     
     norm2 = nc3x*nc3x + nc3y*nc3y + nc3z*nc3z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     nc3x *= inorm;
     nc3y *= inorm;
@@ -193,14 +199,16 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
   
   if (hexaniso_flag) {
     norm2 = n6x*n6x + n6y*n6y + n6z*n6z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     n6x *= inorm;
     n6y *= inorm;
     n6z *= inorm;
     
     norm2 = m6x*m6x + m6y*m6y + m6z*m6z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     m6x *= inorm;
     m6y *= inorm;
@@ -210,7 +218,8 @@ FixPrecessionSpin::FixPrecessionSpin(LAMMPS *lmp, int narg, char **arg) :
     l6z = (n6y*m6x-n6x*m6y);
     
     norm2 = l6x*l6x + l6y*l6y + l6z*l6z;
-    if (norm2 == 0.0) else error->all(FLERR,"Illegal precession/spin command");
+    if (norm2 == 0.0) 
+      error->all(FLERR,"Illegal precession/spin command");
     inorm = 1.0/sqrt(norm2);
     l6x *= inorm;
     l6y *= inorm;