+CHERE IS THE CALCULATION WHICH MIRROR IMAGE IS THE CLOSEST ONE
+ dist_init=(xj-xi)**2+(yj-yi)**2+(zj-zi)**2
+ xj_safe=xj
+ yj_safe=yj
+ zj_safe=zj
+ subchap=0
+ do xshift=-1,1
+ do yshift=-1,1
+ do zshift=-1,1
+ xj=xj_safe+xshift*boxxsize
+ yj=yj_safe+yshift*boxysize
+ zj=zj_safe+zshift*boxzsize
+ dist_temp=(xj-xi)**2+(yj-yi)**2+(zj-zi)**2
+ if(dist_temp.lt.dist_init) then
+ dist_init=dist_temp
+ xj_temp=xj
+ yj_temp=yj
+ zj_temp=zj
+ subchap=1
+ endif
+ enddo
+ enddo
+ enddo
+ if (subchap.eq.1) then
+ xj=xj_temp-xi
+ yj=yj_temp-yi
+ zj=zj_temp-zi
+ else
+ xj=xj_safe-xi
+ yj=yj_safe-yi
+ zj=zj_safe-zi
+ endif
+c print *,xj,yj,zj,'polozenie j'