From owner-chemistry@ccl.net Thu Jul  4 08:54:00 2024
From: "Gary Breton gbreton,berry.edu" <owner-chemistry.:.server.ccl.net>
To: CCL
Subject: CCL:G: How to proceed after a RHF to UHF instability calculation
Message-Id: <-55189-240704085042-28294-wMe7Oitm/T2V7VoD8Vcidw.:.server.ccl.net>
X-Original-From: "Gary  Breton" <gbreton() berry.edu>
Date: Thu, 4 Jul 2024 08:50:40 -0400


Sent to CCL by: "Gary  Breton" [gbreton(_)berry.edu]
Hi everyone,

I'm seeking confirmation that I am performing some calculations correctly.  I 
am running some calculations (i.e., geometry optimizations and frequency 
calculations at BP86-D3/6-311G(d) which has been used before for similar 
calculations) using Gaussian 16 on twisted alkenes.  I am also interested in 
obtaining singlet/triplet energy gaps.

Input for optimizing the geometries and obtaining the frequencies assuming a 
singlet state:

# opt freq 6-311g(d) bp86 empiricaldispersion=gd3bj

I then tested the stability of the final wavefunction for each of the 
optimized geometries:

 # stable 6-311g(d) bp86 empiricaldispersion=gd3bj
 
For some of the more "twisted" geometries, I obtain this result:

The wavefunction has an RHF -> UHF instability

Of course, this isn't surprising since I would expect diradical character to 
begin to mix in as the nominal "p-orbitals" become poorly aligned.

To "fix" the problem, I re-optimized and obtained frequencies using the 
following, again assigning as a singlet:

# opt freq 6-311g(d) guess=mix ubp86 empiricaldispersion=gd3bj

The result was a considerable change in geometry (the C=C bond lengthened).  
Again, I would expect this result if diradical character was mixed in.  The 
wavefunction remains a "singlet" as desired (I am now attributing it as the 
lowest energy singlet state).

I then ran a stability check:

# stable 6-311g(d) guess=mix empiricaldispersion=gd3bj ubp86

and obtain:

The wavefunction is stable under the perturbations considered.

So, I *think* I've done this correctly.  Before I extend this to a series of 
related compounds, I was hoping for some reassurance that this is the correct 
approach.  If not, some direction as to how to proceed would be very welcome!

Best,

Gary Breton
Berry College