virtual double equilibrium_angle(int) = 0;

  virtual void write_restart(FILE *) = 0;

  virtual void read_restart(FILE *) = 0;

  virtual double single(int, int, int, int, double) {return 0.0;}

  virtual double single(int, int, int, int) = 0;

  virtual int memory_usage() {return 0;}

};