Random vector on a sphere

Fortran 90 implementation from Ref. 2. ran() is some randon number generator:

!    The following is taken from Allen & Tildesley, p. 349
!    Generate a random vector towards a point in the unit sphere
!    Daniel Duque 2004

subroutine random_vector(vctr)

  implicit none

  real, dimension(3) :: vctr

  real:: ran1,ran2,ransq,ranh
  real:: ran

  do
     ran1=1.0-2.0*ran()
     ran2=1.0-2.0*ran()
     ransq=ran1**2+ran2**2
     if(ransq.le.1.0) exit
  enddo

  ranh=2.0*sqrt(1.0-ransq)

  vctr(1)=ran1*ranh
  vctr(2)=ran2*ranh
  vctr(3)=(1.0-2.0*ransq)

end subroutine random_vector

References

1. George Marsaglia "Choosing a Point from the Surface of a Sphere", The Annals of Mathematical Statistics 43 pp. 645-646 (1972)
2. Daan Frenkel and Berend Smit "Understanding Molecular Simulation: From Algorithms to Applications" p. 410 Academic Press (1996), algorithm based on:
3. M.P. Allen (Author) and D.J. Tildesley "Computer Simulation of Liquids" p. 349 Clarendon Press (1989)