Subject: Analytical CI gradients in semiempirical programs

From: [email-masked] (Serge Pachkovsky)
Date: Fri, 11 Apr 1997 11:53:29 +0200 (MDT)

Hi everybody,

I would like to use this opportunity to thank everybody who'd offered
to run my simple test job with Mopac93 once more. Now, I would try
to explain the reason for this request, and to pose an additional
question.

Mopac, since version 6.0, implements analytical gradient for CI 
energies. The same code is also included in Ampac (starting from
the version 2.1, I believe).  The implementation is described in 
Dewar M.J.S., Liotard D.A. (1990) J Mol Struct (Theochem) 206:123,
and the theory behind it is quite sound. I wanted to do a few 
benchmark computations with this implementation.

As it was already pointed out on CCL (by Dr. A. Holder,
in his message on 13 Jan 1993), " ... some versions of MOPAC and
AMPAC 2.1 are broken in certain places when it comes to analytical
gradient computation (CI to name just one.) ... ". Indeed, both
Mopac6 and Mopac7 Rev. 1 (which I have locally) have problems with 
the input file given at the end this message. However, it is
Mopac93 Rev. 2 which is supposed to be the latest and greatest
of all Mopacs out there (;-)), so I wanted to try that as well.

Unfortunately, Mopac93 Rev. 2 (thank you, Ernst) still produces
incorrect gradient in my test case, even though it is different
from the result produced by Mopac6 and Mopac7 Rev. 1. This brings
me to my question:

Is there any semiempirical program around which *correctly* implements
the method of the paper referenced above?

With my best regards,

/Serge.P

Here is the input. It does a gradient computation for the third
excited singlet state of a 3x3 CI on closed-shell RHF. (Technically 
speaking, Mopac uses a 4x4 CI, but since one of the states is a
triplet, it doesn't interact with anything and could have been 
eliminated by taking a proper linear combination of configutations
to begin with). The molecule is benzene in almost, but not quite,
D2h symmetry. This should not be a problem, since D2h has no
degenerate representations.

The cartesian gradients on all atoms should be well below 1 kcal/mol/A.
If they are not, then there is a bug in the program you are using.
(This can be verified quite easily by re-doing computation with
NOANCI, which produces correct answer in all Mopacs I know of).

--- cut here ---
MNDO 1SCF GRAD C.I.=2 ROOT=3 SINGLET SCFCRT=0.0000001 &
 PLCRT=0.0000001 T=1000000 MECI

 6          -0.00730   1         1.45978   1        -0.00000   1
 6           1.19790   1         0.77305   1        -0.00000   1
 6           1.19790   1        -0.77304   1        -0.00000   1
 6          -0.00730   1        -1.45978   1        -0.00000   1
 6          -1.21278   1        -0.77327   1        -0.00000   1
 6          -1.21274   1         0.77326   1        -0.00000   1
 1          -0.00698   1         2.55278   1        -0.00000   1
 1           2.16486   1         1.26278   1        -0.00000   1
 1           2.16486   1        -1.26278   1        -0.00000   1
 1          -0.00698   1        -2.55278   1        -0.00000   1
 1          -2.17950   1        -1.26306   1        -0.00000   1
 1          -2.18003   1         1.26315   1        -0.00000   1

--- cut here ---