Subject: Gaussian 98 has problems with large CASSCF spaces

From: Wolfgang Roth <[email-masked]>
Date: Mon, 22 Mar 1999 10:39:46 -0800

Dear all, 

I'm trying to proof the the statement from the subject line because I
observe a strange behavior of the CASSCF algorithm in Gaussian 98 (Rev.
A.4) as reported below:

In brief, if you are going to large(r) active spaces of 10 or 12 electrons
and more than (approx.) 4000 configuration state functions (CSFs) the
energy of the optimized wavefunction is substancially higher (by several
hartree) than for the smaller CAS spaces. My geometry optimizations under
these conditions always failed after a few step due to convergence problems
of the MCSCF wavefunction. 

As an example, I calculated the single point energy of the molecule 2-H
benzotriazole (6 C-, 3 N- and 5 H-atoms) with different active spaces. This
molecule has 10 pi-electrons which should be spread over 9 pi-orbitals such
that the maximum CASSCF space would be designated as (10, 9).

CASSCF( 8, 8,nroot=1) [1764 CSFs, no special selection of orbitals except
for pi] yields
 ITN= 17 MaxIt= 64 E=   -393.5116726512 DE=-7.88D-09 Acc= 1.00D-08 Lan=  3

CASSCF(10, 8,nroot=1) [1176 CSFs] yields
 ITN= 27 MaxIt= 64 E=   -393.4877127352 DE=-9.63D-09 Acc= 1.00D-08 Lan=  3

CASSCF( 8, 9,nroot=1) [8001 CSFs} yields
 ITN= 38 MaxIt= 64 E=   -384.7333360567 DE=-9.20D-09 Acc= 1.00D-08 Lan=  1 

CASSCF(10, 9,nroot=1) [8001 CSFs} yields
 ITN= 18 MaxIt= 64 E=   -379.8942586656 DE=-4.18D-09 Acc= 1.00D-08 Lan=  1

The HF energy at the same geometry is
 SCF Done:  E(RHF) =  -393.396661489     A.U. after   16 cycles

As you can see, the two calculations with the (announced) 8001 CSFs are
about 9 and 14 hartree higher in energy than the HF claculation. THIS
cannot be due to a wrong selection of orbitals.

Did anyone else discovered the same observations using Gaussian 98 or may
it be traced backed to a problem of our computer (SGI Origin2000 with Irix
6.4 /6.5)?

Regards
Wolfgang Roth

==W=o=l=f=g=a=n=g==R=o=t=h===========================W=R==
 University of Leeds
 School of Chemistry
 Leeds LS2 9JT
 UK
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