void init();

  virtual void reinit();

  virtual void setup() {}

  double mix_energy(double, double, double, double);

  double mix_distance(double, double);

  void write_file(int, char **);

                      double, double, double, double, double, double);

  void virial_fdotr_compute();

  // union data struct for packing 32-bit and 64-bit ints into double bufs

  // see atom_vec.h for documentation

  union ubuf {

    double d;

    int64_t i;

    ubuf(double arg) : d(arg) {}

    ubuf(int64_t arg) : i(arg) {}

    ubuf(int arg) : i(arg) {}

  };

  inline int sbmask(int j) {

    return j >> SBBITS & 3;

  }