anchor.diff:      ! Read the PDB and locate each atom record in a grid cube
       READ(PDBFIL, ATMFMT, IOSTAT=IERR) ATOM, AANAM, RESNUM, X
       IF (IERR .LT. 0) EXIT
  -    
  +
       IF(ATOM .EQ. "ATOM") THEN
  -    
  +
           ! Locate atom in a grid square along each axis on at a time
  -    
  +
           DO NX = 1, MX
               XB1 = XMI + BOX*(NX-1)
               XB2 = XMI + BOX*NX
  
positive.code:                EXIT
              ENDIF
          END DO
          
          DO NY = 1, MY
              YB1 = YMI + BOX*(NY-1)
              YB2 = YMI + BOX*NY
  
negative.code: 
      READ(PDBFIL, ATMFMT, IOSTAT=IERR) ATOM, AANAM, RESNUM, X
      IF (IERR .LT. 0) EXIT
  
      IF (ATOM .EQ. "ATOM") THEN
          NATOMM = NATOMM + 1
          XMA = MAX(XMA, X(1))
  