c	combine output from several runs of this using
c	 combhist.for
	CHARACTER*40 INFILE,FILENAME
	LOGICAL EFLG
	real*4 M1(3),d(100)
	INTEGER*4 count(500),I,J,k

1200	FORMAT(A)

	write(6,*)'Enter bin size (AU):'
	read (5,*) binsize
c 	read operator into m2
6	WRITE(6,*) 'ENTER NAME OF pdb file (atom recds only):'
	READ(5,1200) INFILE
	IF (INFILE(:5).EQ.'     ') STOP
	  INQUIRE(FILE=INFILE, NAME=FILENAME,EXIST=EFLG)
	  IF (.NOT.EFLG) GOTO 6
        OPEN (UNIT=2,FILE=INFILE,STATUS='OLD',READONLY)

C	make 1 pass to find minimum z value:
	k=0
	zmax=-1e10
	zmin= 1e10
	
c 	read coord         
50	read(2,51,end=60) x,y,z
51 	format (30x,3d8.3,2f6.2)
c	write(6,*) x,y,z
	if (z.lt.zmin) zmin=z
	if (z.gt.zmax) zmax=z
	k=k+1
	goto 50
60	CLOSE(UNIT=2)
	write (6,*) 'number, zmin,zmax=', k,zmin,zmax
	write (6,*) 'Enter zmin, zmax to use:'
	read(5,*)zmin,zmax

	do 70 I=1,1+ int(zmax/binsize)	
70	count(i)=0

C	second pass to count into bins
        OPEN (UNIT=2,FILE=INFILE,STATUS='OLD',READONLY)
	k=0
c 	read coord         
150	read(2,51,end=160) x,y,z
c	write(6,111) x,y,z
	ibin=(z - zmin)/binsize
	count(ibin)=count(ibin)+1

	k=k+1
	goto 150

160	CLOSE(UNIT=2)
	do 170 I=1,1+ int(zmax/binsize)
170	write (6,*) (i-1)*binsize, count(i)
	END