SUBROUTINE POTV(V,R1,R2,XCOS)

C     TRANSFORM GENERALISED COORDINATES TO THOSE FOR PARTICULAR
C     SYSTEM. THIS VERSION TRANSFORMS TO AB2 BONDLENGTH-BONDANGLE
C     COORDINATES. ALLOWANCE MUST BE MADE FOR THE NUMBERING OF THE ATOMS
c     Input for POTV: R1,R2 are in au. 

      IMPLICIT DOUBLE PRECISION (A-H,O-Z)
c  Dimensions for PES.
      dimension rij(3)
      COMMON /MASS/ XMASS(3),G1,G2
      save icorr
      save icor

C     (R = R . S = R'. T = THETA)
C
      DATA X1/1.0D0/,X0/0.0D0/,TINY/9.0D-15/,X2/2.0D0/

      IF (G1 .EQ. X0) THEN
C        BONDLENGTH BONDANGLE COORDINATES: ATOM 1 = ATOM 2
         Q1 = R1
         Q2 = R2
         THETA = ACOS(XCOS)
      ELSE IF (G2 .EQ. X0) THEN
C        SCATTERING COORDINATES: ATOM 2 = ATOM 3
         XX = R1 * G1
         YY = R1 * (X1 - G1)
         IF (R2 .EQ. X0 .OR. XCOS .GE. (X1 - TINY)) THEN
            Q1 = ABS(XX - R2)
            Q2 = (YY + R2)
            COST = -X1
         ELSE IF (XCOS .LE. (TINY - X1)) THEN
            Q1 = (XX + R2)
            Q2 = ABS(YY + R2)
            COST = X1
         ELSE
            Q1 = SQRT(XX*XX + R2*R2 - X2*XX*R2*XCOS)
            Q2 = SQRT(YY*YY + R2*R2 + X2*YY*R2*XCOS)
            COST = (Q1**2 + Q2**2 - R1**2) / (X2 * Q1 * Q2)
         ENDIF
         THETA = ACOS(COST)
      ELSE
C        GENERAL COORDINATES (INCLUDING RADAU): ATOM 1 = ATOM 2
         F1= X1/G1
         F2= X1/G2
         F12= X1 - F1*F2
         P1= R1*(X1-F1)/(G2*F12)
         P2= R2*(X1-F2)/(G1*F12)
         S1= R1-P1
         S2= R2-P2
         Q1= SQRT(P1*P1 + S2*S2 + X2*P1*S2*XCOS)/(X1-G1)
         Q2= SQRT(P2*P2 + S1*S1 + X2*P2*S1*XCOS)/(X1-G2)
         Q3= SQRT(P1*P1 + P2*P2 - X2*P1*P2*XCOS)
         COST = (Q1*Q1 + Q2*Q2 - Q3*Q3)/(X2*Q1*Q2)
         THETA = ACOS(COST)
      ENDIF
c
c
c
c     Call pes ab initio surface.
       rij(1) = Q1*0.5291772d0
       rij(2) = Q2*0.5291772d0
       rij(3) = THETA
       Call PEScbsfci(VPv,rij(1),rij(2),rij(3))	
       CAll PEScvqz(VPcv,rij(1),rij(2),rij(3))
C       Call PESrel(VPrel,rij(1),rij(2),rij(3))
C       Call pesbodc(VPbodc,rij(1),rij(2),rij(3),31.972071d0,1.007825d0,1.007825d0)
C       Call pesbodc(VPbodc,rij(1),rij(2),rij(3),31.972071d0,1.0075505d0,1.0075505d0)
C       Call pesbodc(VPbodc,rij(1),rij(2),rij(3),31.972071d0,1.00727649d0,1.00727649d0)
C       Call pesbodc(VPbodc,rij(1),rij(2),rij(3),31.972071d0,2.014101d0,1.007825d0)
C        Call pesbodc(VPbodc,rij(1),rij(2),rij(3),31.972071d0,2.014101d0,2.014101d0)
C        Call pesbodc(VPbodc,rij(1),rij(2),rij(3),33.9678669d0,1.007825d0,1.007825d0)
       vp=vpv+vpcv
       if(vp.lt.0.0d0) then
           icorr=icorr + 1
       else
            v=vp 
       end if  
c      Convert to Hartree
      v=v/219474.624d0
      RETURN
      END
C