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Re: Mixing basis sets


Dear Stefan Boresch

For your purposes You can do scanning with help of DLC engine which is implemented in Firefly, please refer to built-in Firefly manual. Some DLC options not included in manual can be found:

Generally, You should add the following strings to your input file:

$CONTRL NZVAR=x $END where x = 3n-6, n - number of atoms in your molecule

and specify by IFZMAT(1) keyword an arrange of atoms that corresponds to:
1. an internal coordinate you want to scan along
2. an internal coordinate you want to freeze

For example, IFZMAT(1)=1,2,3, 3,5,6,8,18 means that you scan along the 2,3 atom pair bond length and freeze dihedral angle between 5,6 and 8,18 atom pairs. Also You must input $SURF group with it's related keywords to specify your surface scanning options.

With such input options You can use cartesian coordinates during scanning and set different basis set for each atom in the manner convenient for You.

On Wed Apr 21 '10 10:03am, Stefan Boresch wrote
>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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