Program APEX4, Version 1.02

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I. Mayer and A. Hamza

Chemical Research Center, Hungarian Academy of Sciences

H-1515 Budapest, P.O.Box 17, Hungary

e-mails: mayer@chemres.hu, hamza@chemres.hu

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This program performs *a posteriori* analysis of the single determinant

Hartree-Fock ab initio wave wave functions produced by the widely used

"Gaussian" system G92, G94, G98, G03. (For DFT-type wave functions

- actulally B3LYP - the HF-like energy of the single determinant built

up of the Kohn-Sham orbitals can be considered.)

The analyses performed:

A) Bond order and valence analysis (I. Mayer, Chem. Phys. Lett. 97, 270

1983; addendum for open shells: Chem. Phys. Lett. 117, 396, 1985, etc.);

B) "Chemical Energy Component Analysis" (CECA), introduced in I. Mayer,

Chem. Phys. Lett. 332, 381, 2000. CECA permits to express the

molecular energy -- approximately but to a good accuracy -- as a sum of

one- and diatomic energy components, the computation of which requires

one and two-center integrals only; three- and four-center effects are

compressed to one- and two-center ones by performing appropriate

projections;

C) The decomposition of the two-center energy components into terms

of different physical origin, as defined in I. Mayer and A. Hamza

Theor. Chem. Accounts 109, 92, 2003.

D) The "Exact Energy Decomposition" as defined in I. Mayer, Chem. Phys.

Lett. 382, 265 (2003). It differs from CECA by the absence of

approximations and a different treatment of the kinetic energy

and the two-center finite basis corrections.

In fact, four different energy decomposition matrices are computed,

two approximate and two exact. They differ in the approximate or

exact treatment of the the 3- and 4-center integrals from one side,

and in the treatment of kinetic energy integrals and (two-center)

finite basis corrections from the other. One type of decompositions

(the approximate CECA and the exact "CECA + 3- and 4-center terms")

contain kinetic energy mainly in the atomic terms (and in the finite

basis corrections) while the "CECA/T" and "exact" decomposition contain

both one-and two-centyer kinetic energy contributions and there is no

need in the finite basis correction terms. Only two of these four have

been described in publications as yet: see CECA in A) and "exact" in D)

above. The diatomic components printed are essentially those in C).

(Plus the kinetic energy is printed).

The "approximate" and "exact" schemes become identical for diatomics.

The CECA-type quantities are good indicators of different interactions,

but are not on the "chemical scale": there arelarge atomic promotions and

large (in absolute value) two-center binding components. The CECA/T and

"exact" schemes give much more "chemical" values, but should be used only

at the equilibrium molecular geometries.

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Installation of the program:

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as a "Makefile", which should be appropriate
to produce the executable

by issuing the command "make" on a UNIX or Linux
system. (If necessary,

replace the call to "f77" in the Makefile by
the name of your FORTRAN

compiler.) The name of the executable generated
is "apost4".

Usage of the program:

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The only input required by the program is the
"formatted checkpoint

file" (Test.FChk) which is produced by the
Gaussian,
provided that the

FormCheck keyword is included in the Gaussian
input. In the program

APEX4 the file Test.FChk is used for input as
the FORTRAN file fort.14,

and is copied to file "Tmp" for actual processing
(file fort.15).

An alternative is to conserve the binary
checkpoint
file which is

produced during the Gaussian run (include
%chk="filename"
in the input)

and to format it by using the command "formchk"
which is a part of the

Gaussian system. The formatted checkpoint file
should then be renamed

(copied) to Test.Fchk (or to fort.14, if the
statement

"open(14,file='Test.FChk')" has been deleted
from "subroutine input".

A convenient way of using the program is to
include
it in the

script performing the Gaussian run; one should
take care of ensuring

that "APEX4" should NOT start before Gaussian
is finished. It is

desirable to delete or rename the formatted
checkpoint
file Test.FChk

after the calculation, in order to avoid its
unwanted reuse.

The program is based on our program APOST. Note that the present

program may be significantly more demanding as for computer memory

and computation time.

Cite this program as:

--------------------------

I. Mayer and A. Hamza, Program "APEX4",
Version
1.0

(Chemical Research Center, Hungarian
Academy of Sciences),

Budapest, 2004.

Limitations:

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in the case of basis orbitals of type f (or higher). In these cases

bond order and valence analysis is still performed.

Bugs etc.

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above - we will try our best to help you.