function overlap(n,h,f1,f2)
c   function to calculate the overlap between two vectors f1 and f2
c      using simpsons rule, assuming f1(0)*f2(0)=f1(n+1)*f2(n+1)=0
c      assumes f1, f2 tabulated at points r=i*h
        implicit real*8 (a-h,o-z)
        dimension f1(n),f2(n)
        if (n.lt.4) then
        overlap=0
        do i=1,n
        overlap=overlap+h*f1(i)*f2(i)
        enddo
        return
        endif
        fac=h/3
        overlap=4*f1(1)*f2(1)+f1(2)*f2(2)
        do i=4,n,2
        overlap=overlap+f1(i-2)*f2(i-2)+4*f1(i-1)*f2(i-1)+f1(i)*f2(i)
        enddo
        overlap=fac*overlap
        return
        end