Some guidelines for running gaussian jobs for Atoms in Molecules analysis

Sample *.com file

1.	# MP2(full)/6-311++G** 6d 10f 
Opt=VeryTight output=wfn density=current  (Other options)

2	H2 opt 6-311++g(2d,2p) 

3	0,1
	H
	H 1, 1.0

4	file.wfn
     
Reasons for each command

1. Route Section:

MP2	Level of theory.  e.g. RHF, MP2, QCISD, etc.  If using post-HF methods 
be sure to include "density=current" (see below). Also you should use the FULL 
option for variational post-HF methods as the default is "frozen core.

6-311++G** 6d 10f	 This can be any basis set, but all AIMpac programs 
analyze the wavefunction file assuming that the basis set contains 6d and 10f 
functions (rather than the standard 5 and 7).  You must therefore always specify 
these basis sets.

Output=wfn	Asks the program to write a file, specifying the wavefunction, 
suitable for analysis by AIMPAC programs.

Density=current is vital when running correlated jobs. Otherwise the SCF (HF) 
density is used for popultion analysis, and more importantly, in the wavefunction 
file.  If all the orbital populations in your wavefunction file are 2.00 you can 
be sure you have a Hartree-Fock wfn. If all the orbitals have energy=0.0 you have 
a natural orbital description.  For HF densities this is fine, but the individual 
MOs are meaningless - try again without density=current, which fixed the problem 
for me.  

2. Title. This information is transferred to the wfn file and then to many of the 
AIMPAC output files.

3. Molecular specification

4. This line tells the program what to name the wavefunction file.  Beware: The 
gaussian version we have sometimes doesn't seem to listen and creates files with 
names like ^D.  You can view these and rename them properly.  The name actually 
given by guassian is listed near the end of the log file. 

Summary of important points

Basis:  6d, 10f
Correlated methods: density=current, Full (except DFT).