Subject: Re: CCL:G:Gaussian 94 IRC calculation bugs?

From: [email-masked] (Mike Frisch)
Date: Wed, 17 Jan 1996 09:31:07 -0500 (EST)

Peking writes:
> 
> Dear netters,
> 	Anybody can kindly point out if it is a bug or I did something wrong?
> 
> 	I used LST and got a saddle point at rhf/6-31g* level, then I try 
> to use IRC calculation to confirm that the saddle point connects my 
> starting materia and the product. however, when I submitted that job, the 
> computer told me that "operation on file out of range" "ER terminatnion 
> in NtrErr". "NtrErr called from FileIO" But I did similar job before using 
> Gaussian92. 
> 
> 	So is this a bug of G94 but not of G92?
> 
> 	Thanks in advance!!!
> 		
>  xiaopeng
> 

This topic is a common source of confusion, which I'd like to
clarify.

The IRC path leads down from a transition state to the minima it
connects.  It starts at a true transition structure, where the forces
are zero and the Hessian has one negative eigenvalue.  The algorithm
in Gaussian also expects to start with the Hessian at the TS.

The LST procedure does NOT locate a transition structure -- it locates
an initial guess for a transition state optimization.  In particular,
it finds a point at which the Hessian has at least one negative
eigenvalue (but perhaps more) and at which the forces are far from
zero.  The results of an LST calculation are not chemically
meaningful, but are only useful to start an optimization to locate the
actual transition structure.

In Gaussian 94, LST is really obsolete, since the Opt=QST2 algorithm
both uses synchronous transit methods to locate a guess for the
transition structure and then goes on to complete the optimization.
This replaces the combination of LST and Opt=TS in G92 with a single
(and much more reliable) algorithm.

The combination of LST and IRC without an intervening Opt=TS in G92
would give a path, but not one satisfying the definition of the IRC
and not one containing the transition structure, so it's probably
better than G94 won't let you do it.  The recommended procedure is
Opt=QST2, Freq, IRC=RCFC.  The Freq step both confirms that the
structure found by Opt=QST2 is a proper first order saddle point and
provides the Hessian read by the RCFC option to the IRC.

Mike Frisch
[email-masked]