CCL:G: Relaxed PES in g03 using forcefield?

Hi Ben,

I list a few facts about the ModRedundant option and optimization algorithms in Gaussian, which might help you sorting out your problem:
1. I advise using redundant internal coordinates for PES scans.
2. There are default optimizers for different levels:
- MM optimizer is used for MM and ONIOM microiterations for the lowest level MM calculation. ModRedundant option DOES NOT work but atoms in Cartesians can be frozen via placing -1 after the atom symbol in the geometry part of the input. At least 3 atoms should be frozen in Cartesians to avoid side effects. There is no option to turn this algorithm on for other methods, Opt=NoMicro turns off ONIOM microiterations and Opt=Large or.Opt=Small will lead to using redundant internal coordinates even for MM calculations to allow the ModRedundant option. Since using redundant internals for very large systems may be costly for MM, an MM/MM ONIOM with microiterations and selecting the atoms which are involved in ModRedundant for the "high" level allows doing PES scans for pure MM calculations. In this case if both levels use the same MM setup the result will be identical to using a simple MM calculation for the whole system.
- Large molecular optimizer is default for semiempiricals, advised for large molecules or ONIOM calculations in conjunction with NoMicro. Opt=Large turns this option on for any calculation, Opt=(NoMicro, Large) is necessary to use it for whole system optimization with ONIOM QM/MM jobs. Opt=ModRedundant works with this method.
- Regular optimizer is usually the default and advised for QM calculations and as the main optimizer for ONIOM jobs. Since only the higher level atoms are treated with this optimizer in an ONIOM QM/MM job by default is is advised to include all atoms belonging to ModRedundant lines into the high level explicitly.
3.    The QM/MM ONIOM calculations use MM microiterations and both, freezing atoms in the Cartesian space and using ModRedundant for the high level can be used.

Best wishes,

Ödön
-
On Tue, 2007-09-25 at 10:23 -0700, Ben King king:-:chem.unr.edu wrote: 
 Sent to CCL by: Ben King [king^chem.unr.edu]
 Dear CCLers:
 Summary:  how can one use gaussian to perform a relaxed
 potential  
 energy scan with a force field?
 We need to generate a 2-D potential energy surface for a
 bond- 
 breaking coupled with a torsion.  Gaussian's tools for
 performing a  
 relaxed PES using the modredundant option to opt are
 ideal, but Z- 
 matrix tools could also work.  Here is the problem:
 Gaussian (g03)  
 completely ignores any constraints when we use molecular
 mechanics  
 (amber, uff, etc.), using either Z-matrices or
 opt=modredundant.   
 This also happens in OMION methods using a MM fragment.
 Our syntax  
 is correct, as we do these same calculations (albeit
 inaccurately and  
 only over limited portions of the PES) using
 semi-empirical methods.
 We must use a molecular mechanics force field: van der
 Waals  
 interaction are important in our molecule and we need to
 study the  
 portion of the bond dissociation region that is poorly
 described by  
 single references wavefunctions (e.g., C-C bond distances
 of 1.4 A to  
 6.0 A in 0.2 A increments). The molecule is large
 (C62H38), so high- 
 level ab initio methods are prohibitively time consuming.
 Molecular  
 mechanics will give us the most realistic description of
 the  
 molecular behavior, provided that we use a Morse potential
 for the  
 bond dissociation.
 If anyone can suggest a procedure to generate a PES from a
 force  
 field, I would be grateful. This is one of those
 irritating things  
 that should be easy but ends up being exceptionally
 difficult.   
 Also,  if anyone knows why Gaussian behaves so oddly with
 MM methods,  
 I would be delighted to know.
 Thanks,
 Ben
 p.s.  The version of Gaussian is: Gaussian 03:
 AM64L-G03RevC.02 12- 
 Jun-2004
 p.p.s.  We are limping through this by running thousands
 of  
 individual constrained optimizations using Tinker, but it
 isn't  
 pretty...
 ----
 Benjamin T. King, Associate Professor
 Department of Chemistry/216
 University of Nevada, Reno 89557
 tel (775) 784-1736 fax (775) 784-6804
 king^_^chem.unr.edu
 http://www.chem.unr.edu/faculty/btk/
 
 
 
 
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 --
 Ödön Farkas
 Associate professor
 Deparment of Organic Chemistry and
 Laboratory of Chemical Informatics,
 Institute of Chemistry,
 Eötvös Loránd University, Budapest
 Address: 1/A Pázmány Péter sétány,
 H-1117 Budapest, Hungary
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 Cell phone: +36-30-255-3111
 Fax: +36-1-372-2620
 URL: http://organ.elte.hu/farkas