CCL:G: Combining Z-matrix with Cartesian coordinates in Gaussian

If you try opt=(modredundant,cartesian)
 You should see the following error at the end of the output file:
  -----------------------------------------
  Cannot AddRedundant with Cartesian or Z-matrix opts.
  Error termination via Lnk1e in l1.exe
 > From the description of your problem, you should be able to use
 modredundant as a method of freezing a bond/angle/dihedral and relax the
 rest of the structures by adding the following 1 space after your z-matrix.
 *Freeze bond between atom 1 & 2:*
 B 1 2 F
 *Freeze angle between atom 1 , 2, & 3:*
 A 1 2 3 F
 *Freeze dihedral between atom 1 , 2,  3, & 4:*
 D 1 2 3 4 F
 This is all with Gaussian 09, Rev. C,  At least for Gaussian, I do not know
 of another way to freeze part of a structure and optimize without using
 modredundant.
 On Tue, Dec 19, 2017 at 10:20 AM, Grigoriy Zhurko reg_zhurko#
 chemcraftprog.com <owner-chemistry(a)ccl.net> wrote:
 >
 > Sent to CCL by: Grigoriy Zhurko [reg_zhurko:chemcraftprog.com]
 >
 >
 > > What are the errors?
 >
 >   Most atoms are frozen.
 >
 > > It's also unclear what you are trying to do.
 > > Are you trying to do a relaxed scan (PES), rigid scan,  or just
 > > optimize?
 >
 > Relaxed PES scan.
 >
 > > If it's a relaxed scan, then opt=modredundant is
 > > required.
 >
 >   Usually opt=modredundant implies fixing (scanning) a distance, angle or
 > a dihedral, but I need to fix a distance between a real atom and a dummy
 > atom between two other real atoms. I don't see how I can specify this
 > constraint with modreduntant only.
 >
 > >  I believe for any modredundant job Gaussian internally
 > > optimizes with a z-matrix only.
 >
 > This is strange for me. I found that Z-matrices are a very bad choice for
 > optimizing rings.
 >
 > > Something along the lines of forcing
 > > the job to optimize using cartesian coordinates, opt=(modredudant,
 > > cartersian), will not work.
 >
 > Are you sure?
 >
 >
 >
 >
 >   Sincerely,
 >   Grigoriy Zhurko.>
 >
 >
 --
 Francisco Guzman
 PhD Candidate
 Otto H. York Department of Chemical, Biological and Pharmaceutical
 Engineering
 New Jersey Institute of Technology
 Email: fg58(a)njit.edu, guzman.research(a)gmail.com