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Geometry optimization in two dimensions


Dear all.

I try to optimize my oligomer molecules without make it twisted (keep it flat) which means being optimize in two dimensions. I try:

1) using cartesian, by input the value of z coordinate cartesian into 0.00000000 for every atoms, then freeze this z of every atoms.

Although manual said that Cartesian couldn't be freezed, I found it on the contrary in another place in this web (‎) and this works for benzene (consists of 18 atoms), but fails for larger molecules (consists of 50 atoms).

2) using z-matrix, by input value of dihedral angles for every atoms into 0 or 180 degrees, then freeze this dihedral of every atoms.

I've tried:
- input every dihedral angles (using format in manual) in IFREEZ, fails when input for 49 dihedral angles, but succeed when input for 1 dihedral angle
- input every dihedral angles in IFZMAT, but it showed error messages "BAD ZMAT INPUT". Also, the DLC couldn't be performed.

I want to ask and ask for any suggestion,
1) Is there any problem in my input? Is there any limitation number of freezing atom/variable? Is there another menu that I could add?
2) Is there another strategy to optimize large molecules and keep it flat (optimize in 2D)? I work with molecules consists of n*50 atoms, the same molecules of other persons on a publication which using GAUSSIAN 03 package. They succeed to optimize oligomer in 2D.

Best regards,

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