Subject: G92: charge-fitting and polarizabilities

From: Daniel Mainz <[email-masked]>
Date: Wed, 6 Jul 1994 16:56:08 -0400 (EDT)

Greetings, Gaussian-ophiles.

We are attempting to use Gaussian 92 to obtain charges to use in a MD
simulation of formaldehyde.  We are running Gaussian on an
IBM RISC-station.

We are having trouble with the population=(chelpg,dipole) option
with the 6-311G** basis.  For some reason, the chelpg routine
thinks that the dipole moment in the z-direction is 13.94D,
rather than the 2.77D it should be.  Furthermore, chelpg
assigns different charges to the two hydrogen atoms of formaldehyde.

A similar problem occurs when we use population=(chelpg),
again with the 6-311G** basis: the molecule develops a net
dipole along the y-axis, in violation of the molecular symmetry.

We have noticed that the 6-31G** basis does not suffer from
this problem, and gives the expected result.  Nevertheless, I would 
like to use a triple-zeta basis set rather than a double-zeta basis.

The same type of problem occurs at the MP2 level of theory, and also using
the scf=(tight,direct) keyword.  Have others out there seen this behavior,
and have quick fixes that we are overlooking? 

Below are the input file and parts of the output file.  Please direct 
personal replies to [email-masked].  Thank you.

Joel Bader and Daniel Mainz, Dept. of Chemistry, Columbia University

Input file:
---------------------------------------------
#RHF 6-311G** population=(chelpg,dipole) scf=(direct) test

formaldehyde experimental geometry

0 1
O
C 1 CO
H 2 CH 1 AHCO
H 2 CH 1 AHCO 3 AD

CO 1.203
CH 1.099
AHCO 121.75
AD 180.

Relevant parts of the output file:
----------------------------------------------
 Dipole moment (Debye):                         /*from the wave function*/
    X=      .0000    Y=      .0000    Z=    -2.7715  Tot=     2.7715
                                             ^ makes sense.
....

 Breneman (CHELPG) radii used for charge fitting
 Generate Potential Derived Charges using the Brenneman model, NDens= 1.
 Grid spacing=  .300 Box extension= 2.800
 NStep X,Y,Z=   19     25     25   Total possible points=       11875
 Number of Points to Fit=    4808

 Charge=    .00000 Dipole=      .0000     -.2127   -13.9414 Tot=    13.9431
              1
  1  O   -1.798223                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  2  C     .519942                        What's going on here?
  3  H     .615452
  4  H     .662829