Subject: Summary: G98 has problems with large CASSCF spaces!

From: Wolfgang Roth <[email-masked]>
Date: Wed, 14 Apr 1999 15:43:30 +0100

Dear CCLers,

about three weeks ago, I posted the following message to this list:

: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)?

I got the follwing answers:

1) From Christopher J. Cramer:
>  I have always found G9x to be inferior to Molcas (best) or GAMESS (very
>  good) for active space sizes beyond about 4x4 -- direct comparisons show
>  G9x to converge to higher energy solutions with alarming regularity.
>
>  This could certainly be the fault of the users (me and my group), but we
>  are reasonably expert with MCSCF calculations -- perhaps the necessary G9x
>  keywords are simply obscure. I note that here at the Anaheim ACS meeting,
>  Gaussian is giving a workshop on using their MCSCF package, suggesting it
>  isn't such a user-friendly component (what MCSCF program is, after all).

2) From E. Tajkhorshid:
>  I had the same experience with regard to the SCF convergence when I use
>  active spaces larger than (8,8). This is not a problem with G98 in my
>  case and I found the same problem also for CASSCF calculations in G94.
>  I've not had problems with energy values, yet. As you mentioned, the
>  active space is selected carefully and cannot be the problem. If you get
>  any answer please let me know or summarize them on CCL.

3) Therefore, Artem Masunov seems to be right when he suggests:
>  Try GAMESS. 1997 version worked much better for CAS then Gaussian94.
>  The 1998 version is supposed to use much less of the disk space,
>  but it crushes on my jobs. Perhaps, you are luckier then me.

Unfortunately, I am also interested in calculated vibrational frequencies
and as far as I know, Gaussian 92, 94, 98 is the only program which is (?)
able to calculate them.

4) The most helpful answer was given by Dr. Paolo Celani after he had run a
set of my input files:
>  I successfully ran your cas(10,9) at sto-3g level. I didn't run all
other cases
>  that you reported in your message.
>  The problem seems to be in the gaussian parser that transforms a keyword
into
>  a set of options. Some of the options were missing or possibly they were
>  conflictual (4/46=1,5/39=000001 must be present if you want to use 'on
fly' 
>  matrix elements).
>  You might want to check this in your output and compare with the output
>  of the test job I appended at the end of this message (in the testjob I
>  explicitly set the options).
>  Read the head of links l405.F and l510.F for more infos.
>  I think it should be possible to use normal CSFs for this job but frankly I
>  don't know how.
>  Since g98 seems to behave differently from the development version
>  I used about 2 years ago, I suggest you to wait for an "official" statement
>  from GAUSSIAN before to post anything to CCL.

The route he suggested is
# casscf(10,9,nofulldiag)
# iop(5/6=8)
# iop(4/46=1,5/39=000001)
# iop(5/16=1)

and solved my problem for the energy and the geometry optimization. But I
could not get the frequency calculation to work even with some of the
keywords given in the head of l510.F.
I tried a lot of options from the information given there but everything
failed.

Unfortunately there hasn't been any answer from Gaussian Inc.

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
===== http://www-public.rz.uni-duesseldorf.de/~rothw =====