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Mixing basis sets

Stefan Boresch
stefan@mdy.univie.ac.at

We plan to scan energy surfaces of N-acetyl-methylamide amino
acids ("blocked" amino acids) to verify (or disprove) "features" found in various force fields. In the case of, e.g.,  Ser, this is a 23 atom / 86 electron system. At both ends of these systems, there are methyl groups that are almost certainly devoid of any interest; however, they cannot be replaced by a hydrogen. Since eventually we may need multiple geometry optimizations on the MP2 level, I would like to use a minimal basis set for these, and a reasonable basis for the interesting part of the molecule.

Now, if I set COORD=UNIQUE, then this is easy and well documented and
the PUNCH file is my friend... However, in the scans of the energy surface I need dihedral angles frozen. This seems to imply using
COORD=ZMT (or ZMTMPC). Is there a way to supply explicitly a basis set
for each atom when using/relying on coordinates in z-matrix format?

Alternatively, is there a way to freeze certain degrees of freedom
if cartesian (unique) coordinates are used?

Apologies if I overlook something trivial in the documentation; as
far as quantum chemical calculations are concerned, I am a complete
beginner ...

Thanks for any hints,

Stefan Boresch

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