CCL:G: keep bond aligned along an axis during geometry optimization in G16

 Sent to CCL by: Frank Jensen [frj~~chem.au.dk]
 I can think of two options:
 1) Fix the Cartesian coordinate of one atom at 0,0,0 using the geom=addredundant
 or opt=addredundant option, place the second atom along the z-axis, and turn off
 symmetry by using the nosymm keyword. This may fail if you are having a 'large'
 molecule and/or not a field along the z-axis and performing an optimization, as
 the optimization can break the z-alignment.
 2) Construct a z-matrix where all your constraint variables are explicitly
 defined, turn off symmetry (nosymm) and use opt=z-matrix to perform the
 optimization in your defined internal coordinates. By using a proper amount of
 dummy atoms (= points in space) and splitting your variables into variable
 (=optimizable) and fixed (= non optimizable) sets, this should do the trick.
 Frank
 Frank Jensen
 Assoc. Prof.
 Department of Chemistry
 Aarhus University
 https://tildeweb.au.dk/au23758/
 https://pure.au.dk/portal/en/frj!A!chem.au.dk
 -----Original Message-----
 > From: owner-chemistry+frj==chem.au.dk!A!ccl.net
 <owner-chemistry+frj==chem.au.dk!A!ccl.net> On Behalf Of Theo de Bruin
 theodorus.de-bruin]-[ifpen.fr
 Sent: Thursday, December 5, 2024 4:25 PM
 To: Frank Jensen <frj!A!chem.au.dk>
 Subject: CCL:G: keep bond aligned along an axis during geometry optimization in
 G16
 Sent to CCL by: "Theo  de Bruin" [theodorus.de-bruin * ifpen.fr] Dear
 all,
 I want to study the impact of an electric field on specific bond a molecule.
 Therefore, I have aligned that bond along the Z-axis, but during the geometry
 optimization, the molecule gets reorientated (-> standard orientation) and
 the bond is not aligned anymore to the axis of the Efield.
 The most intuitive way to correct this, would be to freeze the Cartesian
 coordinates of one atom at (0,0,0) and freeze x=0.0 and y=0.0 for the 2nd atom
 of the bond.
 Yet, Gaussian does not allow to do this, or at least I don't know the right
 syntax for this.
 Do you have any suggestions that conserve the alignment during geometry
 optimizations in Gaussian 16?
 Thanks in advance,
 Best regards, Theohttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtml/http-:-//www.ccl.net/spammers.txt