Gaussian and CAS



 Gaussian Users:
 Steven Bachrach recently noted that the MSRC Ab Initio Benchmark Report
 mentions that we obtained the wrong CASSCF energy from both Gaussian 90 and
 Gaussian 92.  That's true, but I may be able to shed a little more light on
 it.  The particular calculation was intended to exercise each program's CAS
 routines.  It included 4 electrons in 4 orbitals (sigma, sigma*, pi, pi*
 orbitals in ethylene) and used converged RHF orbitals as the initial guess
 for all programs (GAMESS-US, HONDO, MOLPRO, GAMESS-UK, etc.)
 When Gaussian converged to an energy which was higher than the other codes I
 assumed that it was the result of an input error.  After puzzling over it for
 some time, I showed it to several of the other quantum chemists here in the
 MSRC.  No one had any idea what was wrong.  I then contacted Doug Fox via the
 Gaussian Help line.  However, we weren't able to obtain the correct answer.
 Recently Gary Trucks from Gaussian has come up with a suggestion for an
 initial guess that results in the correct energy, but at the expense of a
 large number of iterations.  My understanding is that they're still looking
 into the problem.  Perhaps someone from Gaussian, Inc. can comment here.
 In Gaussian's defense, the benchmark report shows that the program performed
 more of the types of calculations which people are typically interested in
 doing than any of the other codes which were tested.  And, while not the
 fastest in every area, it turned in respectable performance on every platform
 that we had, i.e. SGI, IBM, HP, Cray, DEC, Sun.
 David Feller
 Molecular Science Research Center
 P.S. Copies of the benchmark report are available for mailing.  I encourage
 contributions for the next edition.  There are too many machines and codes
 for me to run them all.