extract() implementation for dihedral opls

doc/src/fix_adapt.rst

@@ -457,6 +457,8 @@ all types from 1 to :math:`N`.  A leading asterisk means all types from
 If :doc:`dihedral_style hybrid <dihedral_hybrid>` is used, *dstyle* should be a
 sub-style name. The dihedral styles that currently work with fix adapt are:
 
++---------------------------------------------------------+----------------+----------------+
+| :doc:`opls <dihedral_opls>`                             | k1,k2,k3,k4    | type dihedrals |
 +---------------------------------------------------------+----------------+----------------+
 | :doc:`quadratic <dihedral_quadratic>`                   | k,phi0         | type dihedrals |
 +---------------------------------------------------------+----------------+----------------+

src/MOLECULE/dihedral_opls.cpp

@@ -431,3 +431,17 @@ void DihedralOPLS::born_matrix(int nd, int i1, int i2, int i3, int i4, double &d
          16.0 * k4[type] * si * cos(4.0 * phi) - 4.0 * k4[type] * sin(4.0 * phi)) /
       (si * si * si);
 }
+
+/* ----------------------------------------------------------------------
+    return ptr to internal members upon request
+ ------------------------------------------------------------------------ */
+ 
+ void *DihedralOPLS::extract(const char *str, int &dim)
+ {
+   dim = 1;
+   if (strcmp(str, "k1") == 0) return (void *) k1;
+   if (strcmp(str, "k2") == 0) return (void *) k2;
+   if (strcmp(str, "k3") == 0) return (void *) k3;
+   if (strcmp(str, "k4") == 0) return (void *) k4;
+   return nullptr;
+ }

src/MOLECULE/dihedral_opls.h

@@ -34,6 +34,7 @@ class DihedralOPLS : 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 *k1, *k2, *k3, *k4;

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

@@ -16,7 +16,11 @@ dihedral_coeff: ! |
   3  56 10 32 84
   4  23 80 61 83
   5  19 90 13 15
-extract: ! ""
+extract: ! |
+  k1 1
+  k2 1
+  k3 1
+  k4 1
 natoms: 29
 init_energy: 2260.6834525285753
 init_stress: ! |-