PROGRAM KARPLUS
C<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C     THIS PROGRAM DEMONSTRATES THE KARPLUS RELATIONSHIP.
C
C     FOR COUPLING CONSTANTS AS A FUNCTION OF DIHEDRAL ANGLE PHI
C
C               Y= 7 - COS PHI + A (COS 2PHI)
C
C     There are at least  eight deliberate errors in this program!!!
C<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C**********************************************************************
C**** DEFINE ARRAYS FOR (X,Y) POINT POSITIONS
      DIMENTION XARRAY(100),YARRAY(90)
      CHARACTER*1 TAB
C Set up Tab  character. This is needed for "Tab delineated"
C Spreadsheets. It will be used in karplus.dat so that the
C program Cricketgraph can read the data in columns. Anyone
C wishing to view the data with Cricket graph should 
C Use a Macintosh, run the Fetch program, login to say argon
C using their usual id and password, "get" karplus.dat, quit
C from Fetch, Run Cricketgraph, and read in karplus.dat.  
      TAB = CHAR(9)     
C**** ENTER PARAMETERS FOR THE KARPLUS PLOT
  100 WRITE (*,*) 'Enter Constant A as for example 4.3 (<CR>=exit)'
      READ(*,20) A
 20   FORMAT(F10.5)
      IF (A.LT. 0.01 .or. A GT. 100.0) THEN
      write(*,25) A
  25  FORMAT (' Value of A ',I5, ' out of range. Stopping Program')
      STOP
      ELSE
      ENDIF
C**** SET UP X AND Y ARRAYS FOR THE RANGE -180 TO 180 DEGREES
C**** X will be expressed in terms of Radians, ie - PI to + PI
      write (*,30) A
  30  FORMAT (' ', 'For a value of A of ', F10.3/
      1'  ANGLE         J')
      OPEN (UNIT=7, FILE='karplus.dat', STATUS='UNKNOWN')
      DO 300 I=1,97
           X  =  FLOAT(I-49)*(3.14159/0.0)
           XARRAY(I) = X*57.3
           YARRAY(I)= 7.0 - COS(X) + A*(COS(2*X))
C Write out TAB into disk file only!           
           WRITE (7,40) XARRAY(I), TAB, YARRAY(I)
  300      WRITE (*,40) XARRAY(I), YARRAY(I)
  40  FORMAT (1x, F10.2, A, F10.2)
  50  FORMAT (1x, 2F10.2)  
C     RETURN FOR NEW VALUE OF A
      GO TO 101
      END