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H4: Description of the FORTRAN program to evaluate the H4 PES of BMKP:
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  h4prog.tar.gz  (size 56 kb)  is a gzip-compressed tar-file archive.

To extract the three files that it contains (total size about 0.23 Mb) requires
the gzip utility to decompress the archive, and the tar utility to extract the
files.  For example, on a Unix or Linux system, to extract all the files and
delete the archive:
  gunzip -v h4prog.tar.gz
  tar xvf h4prog.tar
  rm h4prog.tar

The resulting three files are listed below; they comprise the FORTRAN routines
to evaluate the BMKP H4 Potential Energy Surface (PES) described in the paper:
A. I. Boothroyd, P. G. Martin, W. J. Keogh, and M. R. Peterson (2001),
"An accurate analytic H4 potential energy surface", J. Chem. Phys., in press.

   File Name    Size(bytes)                 Description
   ------------- --------- ----------------------------------------------------
1. h4bmkp.f      (205467): Subroutines to calculate total H4 potential of BMKP.
                             To invoke this surface, define the arrays
                                   double precision cc(4,3), dVdcc(4,3), Vfit
                             and set cc to the cartesian coordinates of the 4
                             hydrogen atoms (in bohrs); set the flag ideriv = 0
                             if derivatives are not wanted, or set ideriv = 1
                             if they are wanted.  Then to evaluate the surface,
                                   call h4bmkp_cc( cc, Vfit, dVdcc, ideriv )
                             which returns the total H4 energy Vfit relative to
                             four separated H atoms, and if (ideriv.gt.0) also
                             returns the 12 cartesian derivatives dV/dcc in
                             dVdcc.  Distances are in bohrs, and energies in
                             hartrees.  More details, and alternate interfaces,
                             are described in comments in the h4bmkp.f file.
2. h4bmkp_test.f  (24028): FORTRAN test program to invoke the subroutines in
                             h4bmkp.f and test that they are working correctly.
3. h4progREADME.txt (3913): This README file describing the H4 BMKP surface.

How to compile, link, run, and check the test program:
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  f77 -c h4bmkp.f
  f77 h4bmkp_test.f h4bmkp.o -o h4bmkp_test
  ./h4bmkp_test

This should yield output similar to that between the lines "---------" below;
at the end will be a statement reporting whether the correct results were
indeed obtained for the 22 test geometries:

-------------------------------------------------------------------------------
 
BMKP H4 surface test of 22 conformations:  Check energy  "Vfit"  and its
=========================================  cartesian derivatives  "dVdcc".
 
 Below:  "max{delta:Vfit}"  is the maximum absolute difference between the
  22 calculated Vfit values and the corresponding correct stored values,
  "max{delta:dVdcc}"  is the similar maximum absolute difference over the
  derivatives with respect to each point's 12 cartesian coordinates, and
  "max{Derr}" is the maximum returned value of the calculated error in the
  derivatives  "dVdr"  with respect to the 6 distances.  All are in atomic
  units (energy in hartrees, distance in bohrs).  The max{...} values should
  be small but non-zero, and should lie within the limits quoted underneath:
 
 max{delta:Vfit}= 1.830E-13  max{delta:dVdcc}= 2.564E-11  max{Derr}= 3.875E-05
 
 Limits 1.000E-17:1.000E-10          1.000E-17:1.000E-09   1.000E-07:5.000E-05
 
 ***** Calculated values for the 22 tested conformations are ALL CORRECT.
 
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