subroutine attach2h(c1,c2,c3,c4,h1,h2,CH,HCH)
c-------------------------------------------
c  Purpose:  Attaches 2 hydrogen atoms to an alkane carbon so that <HCH 
c     and <CCC are mutually bisected by each other's planes.
c
c  Inputs:
c
c     c1,c2,c3,c4  Coordinates of 4 successive carbons.  Hydrogens will
c                  be attached to C3.
c     CH           C-H bond length
c     HCH          <HCH (deg)
c
c  Output: 
c   
c      h1,h2        Coordinates of H atoms attached to C3.
c
c  Programmed by J. Applequist 8/12/97.  Cf. notes JA-H-296-297.
c-------------------------------------------
      implicit real*8 (a-h,o-z)
      dimension c1(3),c2(3),c3(3),c4(3),h1(3),h2(3)
      COTOR(X,Y)=X*DSIN(Y)/DSQRT(1.D0-X**2*DCOS(Y)**2)
      SIBANG(X,Y)=DSIN(X)/DSQRT(Y**2+(1.D0-Y**2)*DSIN(X)**2)
      PI=DACOS(-1.D0)
      STAU=HCH*PI/360.d0
      CSTAU=DCOS(STAU)
      ta=angle2(c2,c3,c4)*PI/360.d0
      cot=cotor(cstau,ta)
      call torang(c1,c2,c3,c4,chi,ierr)
      tors=180.d0-dacos(cot)*180.d0/pi
      bang=180.d0-dasin(sibang(ta,cot))*180.d0/pi
      call chain2(bang,chi+tors,ch,c1,c2,c3,h1)
      call chain2(bang,chi-tors,ch,c1,c2,c3,h2)
      return
      end