In the Utilities subdirectory, there is also a program called "tester.f".  This is a 
routine designed to test the installation of any PES subprogram.  Each PES subprogram 
has within its subdirectory two associated computer files.  If "myPES.f" is the name 
of the PES subprogram, then the two additional files are "myPES.dat" (an input file to 
"test.f") and "myPES.out" (the corresponding output file).  The PES subprogram, 
"utility.f", and "tester.f" are compiled and linked to form an executable file, 
"tester.exe".  When "tester.exe" is executed with input "myPES.dat", it will create an 
output file conventionally named "myPES-test.out".  A satisfactory match of 
"myPES-test.out" with "myPES.out" verifies that POTLIB 2001 was installed correctly by 
the user.  In addition, "myPES-test.out" identifies the reactant ordering (essential for 
determining the non-default input to the array NULBL), the asymptote that sets the zero 
of energy (essential for determining the non-default value of ANUZERO), the largest 
number of electronic surfaces available for calculating the potential energy (essential 
for determining the non-default values of NASURF), and the maximum order of the 
derivatives (if any) which are explicitly calculated by the PES subprogram (essential 
for determining the non-default value of NDER).  Furthermore, other descriptive 
information about the surface, such as a literature reference (if any), will be listed.

Checklist for Installing/Using a PES Subprogram from POTLIB 2001

The correct installation of a PES subprogram involves the steps below.  Assume the 
selected PES subprogram has the name of "myPES.f".

   1.  	Copy "myPES.f", "myPES.dat", and "myPES.out" to a desired directory.
   2.   Copy from the Utilities subdirectory "utility.f" and "tester.f". 
   3.   Compile and link "myPES.f", "utility.f", and "tester.f" and call the 
        executable "tester.exe".  Run "tester.exe" with "myPES.dat" as input 
        and direct the output to "myPES-test.out". Compare the "myPES-test.out" 
        produced to "myPES.out".  If differences in format or round-off error 
        are within machine-specific tolerances, "myPES.f" has been correctly 
        transferred.  

After a successful installation, the correct use of a PES subprogram in a user-specific
application involves the following steps:

   1.   Within the user's routine that will call the PES subprogram, include the 
        following FORTRAN statements:

PARAMETER (NATOM = xx)
PARAMETER (ISURF = yy) 
PARAMETER (JSURF = ISURF * (ISURF + 1) / 2)
COMMON /USRICM/	CART(NATOM,3), ANUZERO, NULBL(NATOM),
	NFLAG(20), NASURF(ISURF+1,ISURF+1), NDER
COMMON /USROCM/	PENGYGS, PENGYES(ISURF), PENGYIJ(JSURF), 
	DGSCART(NATOM,3), DESCART(NATOM,3,ISURF), 
	DIJCART(NATOM,3,JSURF) 

        Here "xx" is the maximum number of atoms, and "yy" is the maximum number of 
        electronic surfaces.  In version 1.0, xx = 25 and yy = 5.  Examining the PREPOT
        subroutine in "myPES.f" will always reveal the most current values for "xx" and 
        "yy".

   2.	If desired, change distance and energy units from the default atomic units by 
        assigning appropriate values to NFLAG(1) and NFLAG(2) in the calling routine.  
        Change the default FORTRAN output unit from 6 by assigning a new value to 
        NFLAG(18) in the calling routine.
   3.	In "myPES-test.out", note the listed ordering of the atoms and change it, if 
        desired, by assigning values to the array NULBL in the calling routine.  
        Similarly, note the listed asymptote for the zero of energy and change the zero,
        if desired, by assigning a non-zero value to ANUZERO in the calling routine.  
        Finally, note the order of derivatives and the number of electronic surfaces 
        available in the PES subprogram and select, if desired, a non-default option 
        for each by assigning values to NDER and NASURF in the calling routine.

	FORTRAN statements:

   4.	Within the calling routine, call PREPOT once as the first call to the PES 
        subprogram.  PREPOT has no arguments.
   5.	Within the calling routine, assign Cartesian coordinates to the array CART 
        using the ordering scheme and units selected above.
   6.	Compile and link the user's application program, "myPES.f", and the utility 
        routine "utility.f".  This will produce an executable file that correctly 
        incorporates the POTLIB 2001 PES subprogram "myPES.f" into the user's 
        application.  For a given CART geometry, each call to POT will then return the 
        energies and derivatives in the specified units, in the specified order, and 
        with the specified zero of energy.