physks.f

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      SUBROUTINE physks(XR,PR,Q,P)
*
*
*       Transformation from physical to KS variables.
*       ---------------------------------------------
*
      IMPLICIT REAL*8 (a-h,o-z)
      REAL*8 xr(3),pr(3),q(4),p(4)
*
*
*       Set the physical distance.
      r2=xr(1)**2+xr(2)**2+xr(3)**2
      r=sqrt(r2)
*
*       Obtain regularized coordinates.
      q(4)=0.0d0
      a=r+abs(xr(1))
      q(1)=sqrt(0.5d0*a)
      b=1.0/(2.0d0*q(1))
      q(2)=xr(2)*b
      q(3)=xr(3)*b
      IF(xr(1).LT.0.0d0)THEN
      u1=q(1)
      q(1)=q(2)
      q(2)=u1
      u3=q(3)
      q(3)=q(4)
      q(4)=u3
      END IF
*
*       Form regularized momenta.
      p(1)=2.d0*(+q(1)*pr(1)+q(2)*pr(2)+q(3)*pr(3))
      p(2)=2.d0*(-q(2)*pr(1)+q(1)*pr(2)+q(4)*pr(3))
      p(3)=2.d0*(-q(3)*pr(1)-q(4)*pr(2)+q(1)*pr(3))
      p(4)=2.d0*(+q(4)*pr(1)-q(3)*pr(2)+q(2)*pr(3))
*
      RETURN
      END