subroutine clarea(t,n,x,y,th)
*     calculate the area and the center of the reactive region.
      integer j,n
      double precision hp,hp2,pi,p2,h,sa,t
      double precision x(0:8192),y(0:8192),th(0:8192)
      double precision tem1(0:8192),tem2(0:8192)
      double precision area,xc,yc
      common/salpha/sa
      common/gridsz/h,hp
      common/valupi/pi,p2

      hp2 = 2.D0*hp
      do 20 j=0,n
         tem1(j) = x(j)*dsin(th(j)) - y(j)*dcos(th(j))
 20   continue
      call fin2(n,h,p2,tem2,tem1)
      area = 0.5D0*sa*tem2(n)
      do 30 j=0,n
         tem1(j) = y(j)*y(j)*dcos(th(j))
 30   continue
      call fin2(n,h,p2,tem2,tem1)
      yc = -0.5D0*sa*tem2(n)/area

      write(13,101) t,area
c      write(14,101) t,xc
      write(15,101) t,yc


 101  format(2(f16.12,2x))

      end