Made virtual agent radius scale factor adjustable by user

src/core/interaction_force.cc

@@ -64,17 +64,19 @@ void InteractionForce::ForceBetweenSpheres(const Agent* sphere_lhs,
   real_t ref_diameter = sphere_lhs->GetDiameter();
   const Real3& nb_mass_location = sphere_rhs->GetPosition();
   real_t nb_diameter = sphere_rhs->GetDiameter();
+  auto* sim = Simulation::GetActive();
+  auto* param = sim->GetParam();
 
   auto c1 = ref_mass_location;
   real_t r1 = 0.5 * ref_diameter;
   auto c2 = nb_mass_location;
   real_t r2 = 0.5 * nb_diameter;
   // We take virtual bigger radii to have a distant interaction, to get a
   // desired density. We assume an interaction distance 3% larger then the
-  // agent's radius
-  real_t additional_radius_scaling = 1.03;
-  r1 *= additional_radius_scaling;
-  r2 *= additional_radius_scaling;
+  // agent's radius by default
+
+  r1 *= param->virtual_agent_radius_scale_factor;
+  r2 *= param->virtual_agent_radius_scale_factor;
   // the 3 components of the vector c2 -> c1
   real_t comp1 = c1[0] - c2[0];
   real_t comp2 = c1[1] - c2[1];

src/core/param/param.cc

@@ -242,6 +242,8 @@ void Param::AssignFromConfig(const std::shared_ptr<cpptoml::table>& config) {
   BDM_ASSIGN_CONFIG_VALUE(diffusion_method, "simulation.diffusion_method");
   BDM_ASSIGN_CONFIG_VALUE(calculate_gradients,
                           "simulation.calculate_gradients");
+  BDM_ASSIGN_CONFIG_VALUE(virtual_agent_radius_scale_factor,
+                          "simulation.virtual_agent_radius_scale_factor");
   AssignBoundSpaceMode(config, this);
   AssignThreadSafetyMechanism(config, this);
 

src/core/param/param.h

@@ -264,6 +264,14 @@ struct Param {
   ///     calculate_gradients = true
   bool calculate_gradients = true;
 
+  /// Scale agent's virtual radius to enable distant interactions.\n
+  /// TOML config file:
+  /// Default value: `"1.03"`\n TOML
+  ///
+  ///     [simulation]
+  ///     virtual_agent_radius_scale_factor = 1.03
+  real_t virtual_agent_radius_scale_factor = 1.03;
+
   /// List of thread-safety mechanisms \n
   /// `kNone`: \n
   /// `kUserSpecified`: The user has to define all agent that must