extract() implementation for dihedral multi/harmonic

doc/src/fix_adapt.rst

@@ -464,6 +464,8 @@ sub-style name. The dihedral styles that currently work with fix adapt are:
 +------------------------------------------------------------------------+----------------+----------------+
 | :doc:`cosine/squared/restricted <dihedral_cosine_squared_restricted>`  | k,phi0         | type dihedrals |
 +------------------------------------------------------------------------+----------------+----------------+
+| :doc:`multi/harmonic <dihedral_multi_harmonic>`                        | a1,a2,a3,a4,a5 | type dihedrals |
++------------------------------------------------------------------------+----------------+----------------+
 | :doc:`opls <dihedral_opls>`                                            | k1,k2,k3,k4    | type dihedrals |
 +------------------------------------------------------------------------+----------------+----------------+
 | :doc:`quadratic <dihedral_quadratic>`                                  | k,phi0         | type dihedrals |

src/MOLECULE/dihedral_multi_harmonic.cpp

@@ -407,3 +407,19 @@ void DihedralMultiHarmonic::born_matrix(int nd, int i1, int i2, int i3, int i4,
   du = a2[type] + c * (2.0 * a3[type] + c * (3.0 * a4[type] + c * 4.0 * a5[type]));
   du2 = 2.0 * a3[type] + 6.0 * c * (a4[type] + 2.0 * a5[type] * c);
 }
+
+
+ /* ----------------------------------------------------------------------
+    return ptr to internal members upon request
+ ------------------------------------------------------------------------ */
+ 
+ void *DihedralMultiHarmonic::extract(const char *str, int &dim)
+ {
+   dim = 1;
+   if (strcmp(str, "a1") == 0) return (void *) a1;
+   if (strcmp(str, "a2") == 0) return (void *) a2;
+   if (strcmp(str, "a3") == 0) return (void *) a3;
+   if (strcmp(str, "a4") == 0) return (void *) a4;
+   if (strcmp(str, "a5") == 0) return (void *) a5;
+   return nullptr;
+ }

src/MOLECULE/dihedral_multi_harmonic.h

@@ -34,6 +34,7 @@ class DihedralMultiHarmonic : public Dihedral {
   void read_restart(FILE *) override;
   void write_data(FILE *) override;
   void born_matrix(int, int, int, int, int, double &, double &) override;
+  void *extract(const char *, int &) override;
 
  protected:
   double *a1, *a2, *a3, *a4, *a5;

unittest/force-styles/tests/dihedral-multi_harmonic.yaml

@@ -16,7 +16,12 @@ dihedral_coeff: ! |
   3  56 10 32 84 52
   4  23 80 61 83 74
   5  19 90 13 15 58
-extract: ! ""
+extract: ! |
+  a1 1
+  a2 1
+  a3 1
+  a4 1
+  a5 1
 natoms: 29
 init_energy: 2854.9857316566695
 init_stress: ! |2-