Remove kinetic energy contribution from virtual sites (#4839)

Fixes #4838

Description of changes:
- fix virtual sites regression introduced in ESPResSo 4.2.0
- update description of `MASS` and `ROTATIONAL_INERTIA` features

doc/sphinx/installation.rst

@@ -442,15 +442,15 @@ General features
 -  ``THERMOSTAT_PER_PARTICLE`` Allows setting a per-particle friction
    coefficient for the Langevin and Brownian thermostats.
 
--  ``ROTATIONAL_INERTIA`` Allows to set the eigenvalues for rotational inertia matrix of particles
+-  ``ROTATIONAL_INERTIA`` Allows particles to have individual rotational inertia matrix eigenvalues.
+   When not built in, all eigenvalues are unity in simulation units.
 
 -  ``EXTERNAL_FORCES`` Allows to define an arbitrary constant force for each particle
    individually. Also allows to fix individual coordinates of particles,
    keep them at a fixed position or within a plane.
 
--  ``MASS`` Allows particles to have individual masses. Note that some analysis
-   procedures have not yet been adapted to take the masses into account
-   correctly.
+-  ``MASS`` Allows particles to have individual masses.
+   When not built in, all masses are unity in simulation units.
 
    .. seealso:: :attr:`espressomd.particle_data.ParticleHandle.mass`
 

src/core/energy_inline.hpp

@@ -305,7 +305,7 @@ inline double translational_kinetic_energy(Particle const &p) {
  */
 inline double rotational_kinetic_energy(Particle const &p) {
 #ifdef ROTATION
-  return p.can_rotate()
+  return (p.can_rotate() and not p.is_virtual())
              ? 0.5 * (hadamard_product(p.omega(), p.omega()) * p.rinertia())
              : 0.0;
 #else

src/core/unit_tests/energy_test.cpp

@@ -17,7 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#define BOOST_TEST_MODULE tests
+#define BOOST_TEST_MODULE energy calculation
 #define BOOST_TEST_DYN_LINK
 #include <boost/test/unit_test.hpp>
 
@@ -69,6 +69,23 @@ BOOST_AUTO_TEST_CASE(rotational_kinetic_energy_) {
         0.5 * (hadamard_product(p.omega(), p.omega()) * p.rinertia());
     BOOST_CHECK_EQUAL(rotational_kinetic_energy(p), expected);
   }
+
+  // virtual particle
+  {
+#ifdef VIRTUAL_SITES
+
+    Particle p;
+#ifdef ROTATIONAL_INERTIA
+    p.rinertia() = {1., 2., 3.};
+#endif
+    p.set_virtual(true);
+    p.omega() = {3., 4., 5.};
+    p.set_can_rotate_all_axes();
+
+    auto const expected = 0.;
+    BOOST_CHECK_EQUAL(rotational_kinetic_energy(p), expected);
+#endif
+  }
 #endif
 }
 

src/python/espressomd/particle_data.py

@@ -178,7 +178,7 @@ class ParticleHandle(ScriptInterfaceHelper):
     rinertia: (3,) array_like of :obj:`float`
         The particle rotational inertia.
 
-        Sets the diagonal elements of this particles rotational inertia
+        Sets the diagonal elements of this particle's rotational inertia
         tensor. These correspond with the inertial moments along the
         coordinate axes in the particle's co-rotating coordinate system.
         When the particle's quaternions are set to ``[1, 0, 0, 0,]``, the