Subject: Ab initio bond orders

From: Istvan Mayer <[email-masked]>
Date: Thu, 30 Mar 1995 12:42:52 +0100 (CET)

Dear Netters,
 
There was a question in CCL about ab initio bond orders. I shall to
summarize here some points which I feel relevant in this respect.
 
Among the programs I know, bond order analysis is CORRECTLY implemented
in MONSTERGAUSS and HONDO-8 systems. (In HONDO-8 the term "bond index"
is used.) It is implemented in GAMESS and Gaussian-92, too, but gives
correct results only for CLOSED SHELL systems (except, perhaps, very
fresh releases of GAMESS). In Gaussian-92 the bond orders are calculated
as a part of the natural bonding orbital (NBO) analysis.
 
The point is in the following. Bond orders and valences for ab initio
wave functions were introduced in my paper I. Mayer, Chem. Phys. Letters
97, 270 (1983), but it became clear very soon that the definition given
there for open shell systems must be modified because, e.g. the bond
order for H2(+) ion was predicted 1/4 instead of the chemically expected
1/2. This modification was made in an Addendum to the paper mentioned
[CPL 117, 396 (1985)], while the problem was discussed in detail in the
paper Int. J. Quant. Chem. 29, 73 (1986) - where the generalization of
bond orders and valences for correlated wave functions was also
introduced.
 
In GAMESS and Gaussian-92 the obsolete formulas of the first paper were
implemented for open-shell systems. At the end of the last year I have
prepared a revised routine for GAMESS which was then rewritten by the
authors of the program in their style - so now they probably distribute
this correct version. The only drawback is that due to some
misunderstanding they have included a RETURN (with no bond order
calculations) for GVB-type wave functions, although calculation for
SINGLET GHF wave functions is also possible, using the same code. (May
be this is also corrected in the meantime?)
 
I have contacted also Gaussian Inc. with this problem, but they refused
do deal with it in a somewhat arrogant manner - except promising that
they will EXCLUDE open shell bond order calculations from their next,
Gaussian-94 program.
 
I am ready to help to those who are not afraid of doing some programming
work by sending them my versions.
 
With best regards to all CCL,
 
Istvan Mayer
 
   Central Research Institute for Chemistry of the
          Hungarian Academy of Sciences
E-mail:
         [email-masked] (or [email-masked])
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