Editing Source code for the minimum distance between two rods

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The following is the FORTRAN source code for the calculation of the minimum distance between two rods in three dimensions. For full details see Ref. 1.

 C  SUBROUTINE TO EVALUATE THE SHORTEST DISTANCE BETWEEN TWO RODS
 C  OF DIFFERENT LENGTH
 C  R12= VECTOR CONNECTING THE GEOMETRICAL CENTERS OF THE TWO RODS
 C  U1 = UNITARY VECTOR DEFINIG THE ORIENTATION OF ROD1
 C  U2 = UNITARY VECTOR DEFINIG THE ORIENTATION OF ROD2
 C  XL1D2= HALF OF THE LENGTH OF ROD1
 C  XL2D2= HALF OF THE LENGTH OF ROD2
 C  RO2 = SQUARE OF THE SHORTEST DISTANCE BETWEEN THE TWO RODS
      SUBROUTINE SDM(R12,U1,U2,XL1D2,XL2D2,RO2)
      DIMENSION R12(3),U1(3),U2(3)
 C *****************
 C STEP 1
 C  SETTING TO ITS VALUE SOME VARIABLES
      R122=R12(1)**2+R12(2)**2+R12(3)**2
      R12EU1=R12(1)*U1(1)+R12(2)*U1(2)+R12(3)*U1(3)
      R12EU2=R12(1)*U2(1)+R12(2)*U2(2)+R12(3)*U2(3)
      U1EU2=U1(1)*U2(1)+U1(2)*U2(2)+U1(3)*U2(3)
      CC=1.-U1EU2**2
 C CHECKING WHETHER THE RODS ARE OR NOT PARALLEL
      IF (CC.LT.1.E-6) THEN
         IF (R12EU1.NE.0.) THEN
             XLANDA=SIGN(XL1D2,R12EU1)
             GO TO 10
         ELSE
             XLANDA=0.
             XMU=0.
             GO TO 20
         ENDIF
      ENDIF
 C   EVALUATION OF XLANDA PRIMA AND XMU PRIMA
      XLANDA=(R12EU1-U1EU2*R12EU2)/CC
      XMU=(-R12EU2+U1EU2*R12EU1)/CC
 C END OF STEP 1   OF THE ALGORITHM
 C ******************************
 C STEP 2
 C   CHECKING WHETHER (XLANDA PRIMA,XMUPRIMA) BELONGS  TO THE
 C   RECTANGLE DEFINED BY THE TWO SEGMENTS IN THE XLANDA,XMU SPACE
      IF ((ABS(XLANDA).LE.XL1D2).AND.(ABS(XMU).LE.XL2D2)) GO TO 20
 C END OF STEP 2 OF THE ALGORITHM
 C *********************************
      AUXI1=ABS(XLANDA)-XL1D2
      AUXI2=ABS(XMU)-XL2D2
 C **********************
 C STEP 3
 C   ASIGNEMENT OF THE SIDE OF THE SQUARE WHERE THE SHORTEST DISTANCE
 C   BETWEEN THE RODS STANDS
         IF ( AUXI1.GT.AUXI2) THEN
            XLANDA=SIGN(XL1D2,XLANDA)
 C END OF STEP 3
 C **********************
 C STEP 4
 C  LOOKING FOR THE LOCAL MINIMA OF THE LINE WHICH CONTAINS
 C  THE SELECTED SIDE OF THE SQUARE
 10         XMU=XLANDA*U1EU2-R12EU2
 C  IF THE LOCAL MINIMUM DOES NOT BELONG TO THE SIDE OF THE SQUARE
 C  THEN GIVE US THE NEAREST CORNER OF THE SQUARE ON THE STUDIED LINE
            IF ( ABS(XMU).GT.XL2D2) XMU=SIGN(XL2D2,XMU)
 C END OF STEP 4
 C ************************
 C STEP 3
 C   ASIGNEMENT OF THE SIDE OF THE SQUARE WHERE THE SHORTEST DISTANCE
 C   BETWEEN THE RODS STANDS
         ELSE
            XMU=SIGN(XL2D2,XMU)
 C END OF STEP 3
 C **********************
 C STEP 4
 C  LOOKING FOR THE LOCAL MINIMA OF THE LINE WHICH CONTAINS
 C  THE SELECTED SIDE OF THE SQUARE
            XLANDA=XMU*U1EU2+R12EU1
 C  IF THE LOCAL MINIMUM DOES NOT BELONG TO THE SIDE OF THE SQUARE
 C  THEN GIVE US THE NEAREST CORNER OF THE SQUARE ON THE STUDIED LINE
            IF ( ABS(XLANDA).GT.XL1D2) XLANDA=SIGN(XL1D2,XLANDA)
         ENDIF
 C END OF STEP 4
 C ***********************
 C STEP 5
 C ONCE WE KNOW THE COORDINATES (XLANDA,XMU) IN THE XLANDA,XMU
 C SPACE WHERE STANDS THE SHORTEST DISTANCE BETWEEN THE SEGMENTS
 C EVALUATION OF THE SHORTEST DISTANCE
  20  RO2=R122+XLANDA**2+XMU**2-2.*XLANDA*XMU*U1EU2
     1    +2.*XMU*R12EU2-2.*XLANDA*R12EU1
 C END OF STEP 5
 C **********************
      RETURN
      END



==References==
#[http://dx.doi.org/10.1016/0097-8485(94)80023-5   Carlos Vega and Santiago Lago "A fast algorithm to evaluate the shortest distance between rods", Computers & Chemistry  '''18''' pp. 55-59 (1994)]