Subject: Summary: ECP´s and Basis Sets

From: Stefan Konietzny <[email-masked]>
Date: Thu, 11 Jan 2001 10:11:16 +0100

Dear CCLers,

thanks to Stefan Fau and Douglas Fox. Stefan´s reply went to the list, but 
not Douglas´. So, here it is.

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Dr. Konietzny,

   I have reproduced the problem of having a zero coefficient with the
CEP-4G* designation actually applies to a number of elements where the
standard polarization functions are not defined.  Through Cl is fine
but I am not aware of a generally acceptable set for the rest of the
periodic table if they were not defined at the point the basis set was
proposed in the literature.  This is the case for the SDD basis sets.

   This is on the list for the developers but I will have to check on
the status.  There was an attempt to query the developers of the various
ECP sets to see what work they had already done.

   You don't say it explicitly so let me make sure you are aware that 
you can augment the CEP-4G basis, or any of the others using the
ExtraBasis keyword which lets you define polarization functions as needed.
Thus specify the route as

# HF/CEP-4G ExtraBasis

supplying manual polarization functions

0 1
Bi  0.0  0.0  0.0
C   0.0  0.0  1.6
H   0.0  0.0  2.6

Bi 0
D  1 1.0
0.75
****
C  0
D 1 1.0
0.8
****

or you can mix/match 

# HF/Gen Pseudo=Read

supplying manual polarization functions

0 1
Bi  0.0  0.0  0.0
C   0.0  0.0  1.6
H   0.0  0.0  2.6

Bi 0
SDD
****
C H 0
6-31G*
****

Bi 0
SDD

where the second entry is for the core potential.  GENECP is implemented
in Rev. A.9 to combine GEN with PSEUDO=READ but the rest of the input is
the same.

  I am sure you will get a lot of recommendations on how to optimize
exponents.  Since the d polarization functions will be formally empty
you either need to use a correlated method or use an ECP where there are
some occupied d orbitals before optimizing the d exponents will work.
Both of these have been used, as you note, and while I would argue for
MP2 atomic calculations as being a good approach which is relatively
close-ended you may come to a different conclusion.

  Using different families of basis sets is also one where most of the
variations have been tried.  I would avoid using a minimum basis type
of ECP and then pairing it with an extended basis set for the first
and second row heavies for the same reason I would not suggest this with
an all-electron basis, it tends to suffer from serious basis set 
superpostion error because deficiencies in the Bi valence basis will be
solved by borrowing of basis functions from the rest of the molecule.

 Let me know if you need further assistance to [email-masked].

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