/***************************************************************************/
/*
  Coded by       : Peter Berczik
  Version number : 1.0
  Last redaction : 2011.II.20. 13:00
*/

int calc_force_n_BH(double m1, double xx1[], double vv1[], 
                    double m2, double xx2[], double vv2[],
                    double eps_BH, 
                    double *pot_n1,  double a_n1[], double adot_n1[],
                    double *pot_n2,  double a_n2[], double adot_n2[])
{

/*				
				INPUT

m1         - mass of the 1 BH 
xx1[0,1,2] - coordinate of the 1 BH 
vv1[0,1,2] - velocity of the 1 BH

m2         - mass of the 2 BH 
xx2[0,1,2] - coordinate of the 2 BH 
vv2[0,1,2] - velocity of the 2 BH

eps_BH     - force softening, can be even exactly 0.0 !

				OUTPUT

pot_n1          for the 1 BH
a_n1    [0,1,2] for the 1 BH
adot_n1 [0,1,2] for the 1 BH

pot_n2          for the 2 BH
a_n2    [0,1,2] for the 2 BH
adot_n2 [0,1,2] for the 2 BH
*/


int k;

double r, r2, r3, r4, RP, x[3], v[3];


for(k=0;k<3;k++)
  {
  x[k] = xx1[k] - xx2[k];
  v[k] = vv1[k] - vv2[k];
  }

r2 = SQR(x[0]) + SQR(x[1]) + SQR(x[2]) + SQR(eps_BH);

r  = sqrt(r2);
r3 = r2*r;
r4 = r2*r2;

RP = 3.0*(x[0]*v[0]+x[1]*v[1]+x[2]*v[2])/r;

// Newton pot + acc & jerks

*pot_n1 = -m2/r;
*pot_n2 = -m1/r;

for(k=0;k<3;k++)
  {
  a_n1[k] = -m2*x[k]/r3;
  a_n2[k] =  m1*x[k]/r3;

  adot_n1[k] = -m2*(v[k]/r3 - RP*x[k]/r4);
  adot_n2[k] =  m1*(v[k]/r3 - RP*x[k]/r4);
  }

return(0);

}
/***************************************************************************/